There is no "signup screen" for "Mastodon." It isn't a centralized platform like Twitter. You choose to follow specific instances or people - and normal people have political views (such as all of the politics in your comment, here.) If you don't want politics, don't follow politics, simple as. Unlike with Twitter, blocking actually works in the fediverse.
But honestly, if seeing one signup form was all it took to send you on a tilt about censorship and radical leftists? Stay on Twitter.
But honestly, if seeing one signup form was all it took to send you on a tilt about censorship and radical leftists? Stay on Twitter.