This sounds reasonable to me - the telephone company has no business filtering phone calls, so it should not filter text messages either. Subscribers may choose to employ a spam-blocking service, which could be provided by other people than the phone company.
I agree with this in principle, but I would add that it is a matter between the subscribers and the telephone company; I do not see that any third party has grounds to interfere in that relationship.
If the telephone company's scheme is optional, subscribers are given a clear choice, and subscribers may change their choice at any time without penalty then I would say it's fine. Failing that, if subscribers have a choice of telcos and at least one of them offers an unfiltered option there is no need for any regulator to get involved.