That's because IRL the context of "but" s doesn't usually go like this when there's a legitimate argument. The point of the comic is not blindly following trans people.
"I support trans people, but X" tends to have transphobic sentiment, even if X could be valid. In your example, many medicines are animal-derived or tested on animals. Focusing on addressing trans prople on this topic is usually a choice.
If you have these views, the context you would usually express them is not addressing trans people, but your message would probably be "I oppose people taking animal-derived medicines even if it improves their health".
Another simplified example. Let's say there's a grifter, scammer, scummy trans person, Alice. If you want to denounce Alice, you'd say "Alice is a scammer, xyz, don't use her as positive trans representation" in a forum, discussion, post, etc. You'd probably not start with "I support trans people, but some of them are scammers like Alice". Imagine how weird that would sound with other minority or opressed groups (women, black, gay, etc).
By the way: I don't know if you're vegan, but most vegans don't share your views on animal-derived or tested medicine (even the Vegan Society definition accepts these medicines). I'd suggest you find another term so you don't misrepresent veganism, like "ethical" or "[strict] animal rights" beliefs. I don't want to invalidate or debate your beliefs but calling it veganism is just not accurate.