why paypal block so many ptc site account,,?

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nowadays,, many ptc site convert its paypal account to alertpay,, they tell us,,"bad news" paypal blocked our paypal account,, so the payment will be give from alertpay,, sorry for the inconvinience,, myloter,, what do you think about it? is it a scam site? or any reason to explain it?? thanks for your comment.. ^_^