And there is no reason for them not to accept it, it's just that their ego would get hurt if they do so, because there is nothing irreversible in Bitcoins...
Bitcoin is a threat to their entire business. Other than that, PayPal has a bank license which in itself will make it nearly impossible for them to incorporate Bitcoin beside all the hassle and legal settlements that they have to go through. Things aren't as easy as some people here like to think.
this is all true but it is only true right now.
in the future i am sure that this will all change and the very existence of processors like paypal would eventually depend on adopting bitcoin and making it as an option inside their systems in order to not be left behind.