"Without a revolutionary theory there can be no revolutionary movement" means that a revolution is achieved with neither verbalism nor activism, but rather with praxis, that is with reflection and action directed at the structure to be transformed. – Paulo Freire, Pedagogy of the Oppressed
Veganism is a revolutionary praxis. It is the reflective-action of non-exploitation. Freire writes that, "if action is emphasized exclusively, to the detriment of reflection, the word is converted into activism" – that is, "action for action's sake." When some use the term "veganism" to denote a diet or consumer activity they are converting the word into action for action's sake.
I believe the reason that veganism is so often stripped of its theory is because it is revolutionary. That is, "without a revolutionary theory there can be no revolutionary movement." Veganism, as an anti-oppression framework that view the abolition of animal exploitation as part of a wider struggle for social justice, is in conflict with (neo)liberal theories. Yet, once disassociated from its theory, "veganism" becomes a hot commodity as a form of activism. A utilitarian will say, "veganism is best viewed as a tool for reducing suffering," and a rights theorist will say, "veganism is the principle of animal rights in action."
I think the appropriation of veganism as activism in service of these theories reflection on their limitations. Freire writes, "When a word is deprived of its dimension of action, reflection automatically suffers as well; and word is changed into idle chatter, into verbalism, into an alienated and alienating 'blah.'" If these theories were more than verbalism they wouldn't need to appropriate veganism to fill in for a lack of action.