In the late 1970s and early 1980s, veganism was undermined by the breakdown of its important relationship between theory and action. We can actually trace this by looking at the publications of the Vegan Society (UK) and the American Vegan Society (US).
In the March/April 1976 issue of Ahimsa, the AVS publication (now called American Vegan), published the article "What is VEGANISM?" on its front page, explaining:
VEGANISM (pronounced VEE-gun-izm) is an advanced way of living in accordance with Reverence of Life, recognizing the rights of all sentient living creatures, and extending to them the compassion, kindness, and justice exemplified in the Golden Rule.
Veganism excludes all forms of cruelty to, and exploitation of, the animal kingdom. Thus, vegans do not use such products of animal cruelty, exploitation, or slaughter, as flesh, fish, fowl, eggs, milk and other dairy items, honey, gelatin, etc.; nor do vegans use leather, fur wool, or silk. Veganism is based on an enlightened sense of the responsibility to other humans and animals ... who share this planet with us, as well as progressive outlook encouraging a healthy, fertile soil and plant kingdom, and a sensible and equitable use of the earth's materials.
There's a clear connection here between vegan theory and practice. The sentence most specifically explaining actions starts, "Thus," emphasizing the action's interrelation to, and reflection of, a principle of non-exploitation. Veganism is not simple activism or action for action's sake.
The following year, in the January/March 1977 issue of Ahimsa, AVS breaks the action-reflection connection by focusing exclusively on action:
Veganism means living on the products of the plant kingdom, to the exclusion of flesh, fish, fowl, animal milk and all dairy products ... eggs, honey, gelatin, and any other food of animal origin.
It also excludes from use animal products such as leather, wool, silk, fur, animal oils, secretions, etc. as may be found in items of clothing, toiletries, cosmetics, household goods, and other commodities, and encourages the study and use of alternatives for all such commodities.
AVS dropped: "and encourages the study and use of alternatives for all such commodities," in April/June 1991. Otherwise, this has basically been the AVS "definition" of veganism for the last few decades.
A similar trend took place with the Vegan Society in England. In 1980, Victoria Moran went to England on a grant to write a thesis on the vegan movement. In her thesis, Compassion: The Ultimate Ethic (An Exploration of Veganism), Moran cites the following from the Society's Summer 1964 issue of The Vegan:
Veganism is a way of living which excludes all forms of exploitation, and cruelty to, the animal kingdom, and includes a reverence and compassion for all life. It applies to the practice of living on the products of the plant kingdom to the exclusion of flesh, fish, fowl, eggs, honey, animal milk and its derivatives, and encourages the use of alternatives for all commodities derives wholly or in part from animals.
Veganism remembers man's [sic] responsibility to the earth and its resources and seeks to bring about a healthy soil and plant kingdom and a proper use of the materials of the earth.
This description contains the same distinguishing qualities as the 1976 AVS description. Both present a dialog between theory and action. Veganism "applies to the practice," it is not a practice by itself.
Moran cites a drastic change in the Spring 1980 issue of The Vegan, which shows the disconnection of action from theory:
Veganism is a way of living on the products of the plant kingdom to the exclusion of flesh, fish, fowl, eggs, animal milk and its derivatives (the taking of honey being left to individual conscience). It encourages the study and use of alternatives for all commodities normally derived wholly or partly from animals.
In her thesis, Moran wisely settled on neither vegan societies' narrow action-oriented description; opting instead for Eva Batt's description in "Why Veganism," stating that "veganism is one thing and one thing only – a way of living which avoids exploitation whether it is our fellow man [sic], the animal population, or the soil upon which we all rely for our existence." (Batt was connected with both society's. Her article "Why Veganism" was published in several forms by AVS, and she worked in several positions for the Vegan Society.)
What's happened to veganism is a lot like when whole foods are processed and stripped of vital nutrients. Veganism as both theory and action is the whole grain, but as just a practice becomes refined white flour. The adoption of veganism by utilitarian and rights theorists becomes enriched white flour. Through the process of "refining," veganism, like whole grain, becomes a denatured commodity. An animal rights theory might add something to the refined product, but it's not the same as whole, organic veganism.
With regard to The Vegan's Spring 1980 description of veganism, the Vegan Society only left honey to "individual conscience" during the early 1980s. The current description used by the Society excludes all animal-derived products including honey. It also returns some reference to non-exploitation as the basis of the practice, and gives consideration to other people and the planet.
The American Vegan Society website does a better job describing veganism as more than just a practice (American Vegan still uses the narrow, action-oriented definition). Unfortunately, AVS doesn't identify exploitation as a fundamental concern, which I think can be problematic.