Veganism entails not eating meat/dairy. It doesn't mean you have to eat healthily, it doesn't mean you can't eat junk food, it doesn't mean you have to be held to a higher standard than others.
No one's saying you can't eat 'junk' vegan food. Of course you can. What some are saying is that it seems a bit unethical for a professed vegan to actively support to the profits of a massive meat food chain like McDonalds.
It doesn't mean you have to eat healthily
But this is what is often pushed as a reason to switch to a vegan diet.
it doesn't mean you have to be held to a higher standard than others
But it does. Because this is the only reason you can sell it to me. You cannot sell it to me on health grounds. You have no way of proving that veganism is superior to even a high fat low carb diet, or the Cave Man/paleo or blood type diet. Taking a brief look at the healthiest countries in terms of diet you see plenty of fish, some meat, veg, dairy, complex carbs and - paradoxically in some - quite a lot of saturated fat.
So the only way you can sell it to be is if you take it all the way, you hate the exploitation of animals and won't support anything that does. Ok fine. I disagree for now, but I can buy that because it makes sense.
This half-foot-in-half-out approach as far as the philosophy is concerned doesn't persuade me.
Put it this way, I don't agree with wearing real fur. Would I shop at a chain that produces apparel made from real fur, even if they also sell clothes made from man made fibres, even clothes I really like?
No.