Is it rude to ask for a tip?

Supplier REQUESTS a tip and insults us because we refuse? Reason is below please read?

Hey

So yesterday I was eating Asian with my boyfriend and his brother. You can find whatever you want, but both are very respectful men, always friendly to the waiter, for example, but don't see the "usual" tip - since in their opinion the waiter is "paid for what he does" , where you are right somewhere and I understand that if someone just doesn't want to tip a € 2 tip. I sit in the office, make an effort and don't get any money from my customers on the phone for whom I do things, for example.

Then yesterday, when we paid, my friend's brother wanted to pay with an EC card, the amount was € 30.20, he said "make it € 30.50", i.e. tip 30 cents. The waiter was of course annoyed by us, gave us the receipt from the ec device without goodbye etc. and disappeared, while my brother-in-law suddenly noticed that the waiter had probably debited / typed 32 € without asking. He said he wasn't going to make an elephant or anything out of it and left it at that, but I was really amazed at how you could dare ... justified? Suppose someone is living on extremely little money a month because of XY and is dependent on every penny, I thought, for example. That's not clear, is it?
And please do not make any comments about their handling with tips. As I said, they are always polite and friendly and it is up to them to decide how much and whether to tip at all. Thanks. :)