Cell carriers want it coming and going
In the U.S.A. cell companies, and their landline counter-parts, are common carriers and therefore protected by law from the content they carry. This also applies to ISP's in that country, which is why it always best to host chat-rooms there - hosting is libel free.
T-Mobile, therefore. is clearly making a judgement call. Telephone companies give service to anyone with money and therefore T-Mobile should, too.