I don't live in the US, but I have heard of this tipping culture and it's becoming a must every time people will pay for dine in or avail for such services.

But it seems that most workers are expecting tips even if the customers are not comfortable giving that to them and hasn't received good service from them.

It should be out of kindness and voluntarily because they're paid to do their job and whoever started it making it as a responsibility as a customer, well, it's on you folks if you're feeling that you want to give the tip, give it. However, it should not really be a mandatory.