When you’re the most successful company in any given field, it’s only natural that people are going to take shots at you.
And especially if you’re talking about creators who are self-publishing, they’re not going to have the resources nor the pull that a big outfit like Marvel possesses.
It’s also a very tried-and-true tactic. Everybody loves an underdog. And everybody loves the little guy who “stands up” and “protests” the corporation by doing things their own way.
And sometimes that works out. And sometimes it doesn’t.
And sometimes, that person eventually ends up doing work for a big company like Marvel at some point.
I don’t think it’s especially unprofessional. It’s more a question of whether it does them any good or not. In the end, it’s the content of their work that’s going to be the most important thing. You’re not going to keep readers for very long simply by railing against Marvel (or DC) if your material is weak.
There’s room in this industry for all kinds of comics.