I'm pretty sure it's just a joke. There's no way such a thing would be enforceable.

Sure but this is the type of thing that shies corporate lawyers from approving usage of something.

"We can't guarantee we'll be nice ..."

See also "don't use for evil" clause in JSON library by Douglas Crockford and a special exemption for IBM: "I give permission for IBM, its customers, partners, and minions, to use JSLint for evil." [0].

[0]: http://dev.hasenj.org/post/3272592502/ibm-and-its-minions

More specifically "define 'nice'"

It is a joke but a stupid one that harms adoption of the product. It's conceivable the author could decide say, eating meat isn't "nice" and sue an organization using it that packages meat products for consumption. A judge may find it unenforceable but having to potentially get tied up in a legal battle in the first place already makes it unpalatable to many organizations. See one of the replies here about a similarly juvenile clause where organizations got written permission to use a product "for evil". Not sure a few giggles is worth harming the adoption of technology many people have worked hard to deliver.

You can bring up sad and negative topics without being an asshole. That means you are being nice.

For example, you are not nice if you say:

trumptards do x,y

You are nice if you say:

x,y is not good in my opinion, because (list of arguments without resulting to offensive stuff)

Certainly, in decades of commenting you'll never find a comment where I call someone a foul name.

...but we're just hoping at this point that they use your definition of "nice" and not the version I gave where having "bad" opinions is "abuse".

I know what you mean.

I had this issue before. I was complaining about a product on a forum and was a bit annoyed because it was expensive and did not work properly. I didn't call them names, but it was obvious in my tone that I was dissatisfied. Sadly, the moderators on that forum called it "trolling" and I got banned. Those moderators sadly also made money through the company I complained about since the forum is linked to the store.

So yeah. I can see what you mean.

I think the broader point is that (pseudo-)legislating politeness is bad policy.