Paypal Website Payments Pro is very similar to a true merchant account and payment gateway in the US except PayPal is providing both services instead of each component being separate (which is typical in the US).
Just like a true merchant account and payment gateway there are limitations to where payments may originate from. This is almost entirely due to risk, specifically fraudulent transactions. A true merchant account in the US typically limits transaction to the US and Canada unless arrangements are made during the account setup and, even then, it's rare to have an account that allows transactions outside of those two countries without the rates going up significantly.
Paypal is essentially placing the same restrictions on their merchants except they are a bit more flexible/different in these ways:
If you are located in the US or Canada (the latter does not have access to US merchant accounts under normal circumstances) you can accept payments from Canada, the United Kingdom, or the US.
If you are in the United Kingdom (where you cannot normally get a US merchant account) you can accept payments from anywhere but only if they are in the Visa brand.
Number 2 is most likely because Paypal is using a different acquiring bank for their US and Canadian merchants. This is because US and Canadian merchants accounts operate in a very different, and usually lower cost, way then International merchant accounts.