They really don’t... they’re a private business. They owe no one an explanation
You are technically correct, they aren't required but its a good business practice.
For example I rarely read all the terms and conditions (I know I know) on websites like that. If it wasn't for this thread I wouldn't have known using a VPN was an issue, now I know not too. Assuming that is the problem? That would have been a problem for me.
If you don't educate your customers they will continue to be dummies.
Again though, I'm not on high horse demanding they explain themselves, just seems like potential customers have the right to know why service was denied to them. If nothing else so they can tell their friends and family.