Hi diamond, and Welcome to Cruise-Chat!
According to Royal Caribbean Reservations you should be able to book the Oasis part without any problem with a US Agency. Now, the Princess part is the problem. As Dave noted, this is because of agreements, rules, and policies that are currently in place. It is sort of a nutty thing from my point of view, but it is what it is.
If you are traveling with US or Canadian citizens, US Princess Agencies can book it as long as the bookings are cross referenced. I will forward the International Travel Agency email that Princess recommends via a Private Message just in case.
Again, I think it is a "nutty" rule and Welcome Aboard,