The issue is far more complicated than some seem to believe. On one hand, this is pitched as an infringement of "free speech" rights. On the other hand it is seen as within the purview of any ad network or content publisher to decide what it will run, and what it will not run.
In other words, it is a manifestation of an ancient debate: Does the right of political speech belong to the speaker or the listener?
Also, though communications is regulated on a "common carrier" basis, which implies no "right of free speech" for the carrier (no blocking, for example), a commercial text messaging campaign is the use of a common carrier capability for an advertising campaign.
Historically, content publishers and media have had the nearly absolute right to reject any discrete bit of advertising, exercising their "free speech" rights.
These issues are not easily resolved. One might argue it is clear enough that the original intent of the U.S. Constitution was to preserve the right of free political speech for "speakers," not "listeners," originally for publishers of political tracts, pamphlets and the forerunners of newspapers.
More recently, the rules have shifted for electronic forms of communication, putting new emphasis on the "rights" of listeners or viewers. That's the foundation of all "local content" or "diversity" rules, for example.
But those interpretations conflict. Is the right of free speech primarily or necessarily for the speaker, or for the audience?
Secondarily, in this case, is texting a form of media, and therefore protected by free speech rights, when the use case is "advertising and marketing" rather than person-to-person communication of the common carrier type (private conversations and messaging)?
If the former, then T-Mobile has the right to reject an ad; if the latter, then all the messages must be delivered.
These sorts of questions have gotten more complicated as media have evolved, but there is a basic contradiction here, nonetheless. Is the right something that belongs to the speaker, or the listener? Courts have ruled both ways.