>>Because there is no alternative and they fucking know it.
Off the top of my head, https://payments.amazon.com/ is a great alternative. Not sure how the rates stack up, but I definitely trust Amazon more than I trust PayPal. I'm sure there are more alternatives available, I don't know why so many people insist on using PayPal. I think Amazon allows donations cause kickstarter.com uses it, and you don't necessarily get anything if you fund a project. I'm guessing the tricky part here is the "donations", but PayPal certainly has a poor customer satisfaction record.
From Comcasts AUP:
What will happen if I exceed 250 GB of data usage in a month?
The vast majority - more than 99% - of our customers will not be impacted by a 250 GB monthly data usage threshold. If you exceed more than 250 GB, you may receive a call from the Customer Security Assurance ("CSA") team to notify you of excessive use. At that time, we will tell you exactly how much data you used. When we call you, we try to help you identify the source of excessive use and ask you to moderate your usage, which the vast majority of our customers do voluntarily. If you exceed 250GB again within six months of the first contact, your service will be subject to termination and you will not be eligible for either residential or commercial internet service for twelve (12) months. We know from experience that most customers curb their usage after our first call. If your account is terminated, after the twelve (12) month period expires, you may resume service by subscribing to a service plan appropriate to your needs.
They say they will help you identify the reason you went over your cap. "When we call you, we try to help you identify the source of excessive use and ask you to moderate your usage, which the vast majority of our customers do voluntarily." Which they clearly didn't do in this case. This would be acceptable if we had another option for broadband internet, but we don't.