I think you are right on the money. In addition to forum posts I've seen several exchanges on the freenode #dreamhost channel that seem to verify that certain fraud screening takes place before a payment page is even offered.
DH steadfastly declines to confirm or deny that, or how, this is done -but it seems pretty clear that they are deriving some info from the IP address of the user, and possibly comparing that with user information entered, as an initial screening
To me, it seems quite likely, and reasonable, that limited payment options are offered to some users based on this information. Proxy users, where their customer data cannot be seen to bear a geographical relationship to their IP address, those whose IP address does not appear to be probable for the address they have provided, etc. are, in my opinion, likely to see such limited payment options.
This appears to be a successful part of their fraud deterrent system, and they repeatedly answer queries regarding the particulars with the statement that they "will neither confirm or deny" that a particular test is made, and will not discuss exactly what triggers their fraud alert status (a position that, by the way, I fully support)