in the US, CDMA operators (notably Verizon) typically do restrict unblessed devices (where, in many cases, those include supported devices simply not sold by them). this is improving, but remains common.
the situation for GSM operators is somewhat better. the GSMA regulations (the industry association every notable global operator is a member of) provide at least some protections for users to ensure device portability. the GSMA allows north american operators more latitude than most, and the GSMA-NA is permitted to, essentially, override many portions of the regulations. i'm not sure if they've done so on this point. certainly from the point of view of a user in north america, the GSM operators are perceived as open whereas the CDMA operators are (up until about a year ago, when this started to soften) perceived as very closed.
