Ignoring the amount customers confirm is no security bug according to PayPal
This is how it has always been; it's written in the documentation. I don't personally consider this a bug, since a retailer could feasibly accept a credit card and charge whatever they want to it. The fact the PayPal allows the amount to be changed is not dangerous, because PayPal holds the liability and any charges can be reversed. Furthermore, the business who charges consumers without consent will be committing fraud.