Dystopian fiction has given so much terrorizing weight to the concept of "thoughtcrime" and "wrong think" that I think people forget how much
positive effect a culture can have on widespread mindsets and stopping ignorance.
There are some ignorant thoughts that have almost entirely fallen out of the collective consciousness (in terms of serious belief, not abstract discussion of the past). (As a random example, there are vanishingly few ignorant people left who genuinely believe that disease is a punishment of the wicked that should not be interfered with, "ostracize the lepers", etc.) There are many more that need to.
If you think that some people are inherently "lesser" than others and can be treated as such, that's wrong. If you think that it's OK to assault people, that's wrong. Whether that turns into violent action or not, it still pervades other actions and societal structure, and it's the source of more subtle discrimination; that it has gone as far as being written and conveyed in ways that influence others makes it more than just thoughts. It's a good thing to influence society in ways that make people who think and write and talk and influence others in that way less successful, favoring people who don't, and over time find that fewer people think that way.
Because the same power that makes it possible for institutions to do that also gives us a check on those institutions. It seems entirely fine for an institution to look at the kind of person they're accepting and be selective in this way. If we start seeing institutions be selective for things we don't want to see selection on, we can push back on those institutions. There's no slippery slope here.
We're not talking about violent suppression of free speech. We're talking about non-violent exercise of the right to free association, to shun racists and sexists.