It's not tongue in cheek. You described veganism as a dietary preference, but it is an ethical belief and practice. Keeping silent in the face of unethical behavior is normally seen as cowardly or, at the least, not a general positive. If you came upon a person kicking a child, you would likely want to intervene, not merely think to yourself that you wouldn't do the same.

Here's a short medium post that sums it up decently, quoted for your convenience:

Look through the comments of the latest Facebook post that has aroused the ire of non-vegans and you’re bound to see the following:

‘I don’t care if you’re vegan, just respect my choice not to be vegan.’

‘How come a vegan gets mad if you serve them meat but won’t serve you meat.’

Stop telling other people how to eat!

These reactions to the promotion of veganism and vegan food products would make complete sense if veganism were a dietary preference, akin to trying not to consume sugar or not liking pickles. But veganism is an ethical stance against the commodity status of animals. Following a vegan diet results when you follow this philosophy.

When you understand the vegan philosophy, the idea of respecting someone’s choice to needlessly consume animal products no longer makes sense. Advocating that people stop using animals to the greatest extent possible is the ethical thing to do, and condoning animal use immoral. Serving non-vegan food to a vegan is rude but serving vegan food to a non-vegan is acceptable because, while a vegan has a moral conviction against eating animals, a non-vegan doesn’t believe that eating a meal without animal products is unethical. They’ve likely eaten Oreos or peanut butter on toast many times.

https://medium.com/fiercely-gentle/why-do-people-still-think-vegan-is-a-diet-9ae2ec6213e8