Read the PayPal agreement, they have the ability to close your account without notice for anything they feel doesn't follow their Terms Of Service.
I had an account with PayPal, which they summarily closed without notice and froze my funds for 180 days. Yet I also have email from them saying my business is fine and doesn't violate their TOS.
There is no way to appeal their decision either, get a real merchant account and you'll be happier. Merchant accounts have lower fees and you are paid directly.
The other systems that were mentioned are third party payment systems. Which means someone pays them, and then they pay you. In the case of PaySystems, who was a third party payment system, they were paid, and then closed down without paying their merchants.
I'm not saying it's going to happen or will happen, but it's your money and your business and your customers you need to protect. Investigate all of your options and choose what you think is best for you and your customers.