Well, the same goes for their regular fees. While I know there is a transaction fee, you never see it next to the transaction amount until the final statement. The same with third-party ATMs: they'll warn you about the fee they are going to charge, but not about the fee your bank is going to add on top of that. Even your own bank's ATM will slap you with a fee when you withdraw money from your credit card, but you won't know about it until you get the statement (when it is already too late to reconsider the transaction).
In other words, nothing particularly exceptional or sinister about PayPal practices. In all of the above cases, it is all disclosed somewhere, usually in small print, but not during the transaction. Not that I condone it, but I file it under "life's little annoyances." Teams of marketeers, psychologists, economists, behaviorists, spent decades perfecting methods meant to induce you to spend more by hiding the true cost of it, by making it look smaller than it is. My pet peeve: after ten years in the States, I am still royally annoyed that my restaurant bills come up 25-30% higher than the sum of menu prices (tips and tax).