As a 1st Amendment superfan, this is a bad take. The relationship between DTRPG and their customers, both providers and buyers, is contract driven. And that clause does not implicate the freedom of speech in any way.
Well, I noted before that this isn't related to the First Amendment, which is concerned with the legal principle of the government not restricting freedom of expression, which is neither here nor there where DTRPG is concerned.
Likewise, the contractual nature of the relationship between DTRPG and publishers needs to be viewed, I believe, in the broader perspective of the fact that, in a very real way, OneBookShelf controls access to most of the potential market. While they might have the legal right to therefore exercise that through a clause which demands that anyone who wants to publish on them therefore cannot speak ill of them (i.e. make derogatory comments), I'm of the opinion that doing so doesn't serve the greater public interest...and that they're large enough within their market to where this should be something that they
have to take into account, at least within the context of not screening out publishers based on their personal opinions.
As for your general argument about free speech and consequences, I'm almost 100% positive that you would not support that someone making strident racist remarks must be fully accommodated by a member of the race they are demeaning in all ways, including continuing to associate with them, do business with them, and refraining from saying anything bad about them in return. This is the outcome you're arguing.
As I noted above, I'm of the opinion that when a single entity has outsized control over access to a particular market, they assume a burden whereby they're no longer allowed to simply look out for their own interests. In that case, they might very well find themselves obligated to interact with people whom they personally disagree with, don't like, and even find reprehensible. But that doesn't obviate that burden that their outsized influence has put upon them.
I get your point, though, you think that a big company like DTRPG (heh, big) that has a large footprint in a market should not be able to say things like "don't defame us or we cut you off." To you, this feels like they're telling you what you can and can't say. They aren't. You can still say whatever you want. But just like you're free to say whatever you want, they are free to end their voluntary association with you. That you might or might not rely on income based on that voluntary association, or that you might or might not have other viable options for that income, is entirely beside the point. Your rights do not remove the same rights of others.
I don't believe that last part, wherein your income is affected, is beside the point. DriveThruRPG's rights aren't being curtailed because they conflict with yours, but because they've accumulated sufficient power within their particular market to where it becomes necessary for them to serve (in certain capacities) the entirety of that market, rather than themselves. At that point, I'm of the opinion that they can't simply cut someone else off for personal reasons, since doing so makes it clear that they're now gatekeeping (most of) that market from those who want to access it.
Now, I know that comes across as hyperbolic, since the downloadable RPG product "market" is puny, and compared to transnational conglomerates and global corporations OneBookShelf is a tiny little company, but the principle stands regardless of the scope it's applied to.
EDIT: That said, I want to take a moment to personally thank you for the erudite and respectful reply. I think that posts like yours are an excellent example of the level of discourse that makes EN World a refreshing change from how these conversations usually go on most of the Internet.