Yes, that's the case; however, which category that gets chosen for the payment amount is somewhat random (well whichever $IN->param() returns first). So if you have categories which do not require payment, and one of them is chosen, then the user may be able to add the links without payment if $IN->param() returns that category as the first one.

If you have a payment-is-required site and don't want to charge per category the link is in, then allowing multiple category inserts should work fine for you.

Adrian