I understand your resentment. The only thing I can say is that pretty much all credit card companies work in the same manner. Credit card companies as a whole are ALL a bunch of crooks. They are a big part of the reason that this countrie's economy is in the shape it is. The best thing you can do is either not have them or not have any balances on them.
I'm sure it might not have been your fault at all. The way most of them work, even it is a mail delivery delay, and it gets there a day late and perhaps it was only the first time this has happened to you; they boost your interest rate. It seems unfair but I just wanted you to know it's not just Discover.
I notice a lot them now are continually lowering your credit limit. Say you have a $1,000 limit. If you get it paid down to $500, then they drop your credit limit to $500. They even do it to oddball amounts too. If you get your balance down to $413, they can drop your limit down to $413. And it doesn't matter if you have NEVER been late. They have the right to run a periodic credit check. So beware of this happening on your other cards. Because of your issue with Discover, you could easily see this happening with some of your other credit cards.
Plus, this will hurt your credit score since your credit score is not only calculated by whether or not your payments are on time but also by how much 'available' credit you have. So if you owe $500 on a $1,000 limit card and they drop the credit limit to $500, your credit score just took a nose dive. On the other hand, if you have a $1,000 balance on a $1,000 limit credit card and you can get them to raise your limit to $2,000, then your credit score will go up.
Again, I know it's screwed up but I just thought that might be some helpful information to those of you out there that don't know any of this information.