Yes, it actually is. No one eats turkey bacon or beef pepperoni because they fundamentally disagree with eating meat.It's no more dishonest than turkey bacon or beef pepperoni.
People have different reasons to be vegetarian. If you don't believe in killing animals or your religion prohibits certain foods but still want to eat a cheeseburger, here you go. The Filet-o-fish exists because Catholics exist. It's a fish based burger substitute, giving people who could not eat a burger on certain days an option to spend money on a different sandwich instead. Do you sit around and wonder why people eat fish but won't eat beef on a given day and why they have to consume it in a burger-esque setting? Probably not. What's it to you what a vegan eats or doesn't eat? Do we really need to tribalize based on menu choice now?
If you don't believe in killing animals but want you're food to emulate dead animals, you have a major flaw/disconnect in your way of thinking. If your religion prohibits certain foods, then is seeking imitations of those forbidden foods really in keeping with the religion? I can't imagine that it is in most religions. I am a Catholic and no, a fish sandwich is not a burger substitute, it is a convenient, handy way to eat fish. Just like any other sandwich is convenient, handy way to eat whatever is contained between the bread. No, I don't sit around wondering that because I know the reason. Frankly, I could sit around and wonder why you insist on repeatedly trying to interject religion into a conversation about vegans and the fake meat marketed almost exclusively to them but I'm pretty sure I already know. A vegan is free to eat whatever they want and to hold and express whatever opinion on the subject they want. However, I'm equally free to express my opinion on it as well. The real question is who are you are to tell me or anyone else we can't express our opinion on the topic? Personally, I think we need more conversation and fewer attempts to stop those from sharing opposing views. I'll leave the silly trope of tribalism as a bogeyman for another thread.