SMS is unencrypted and routes through the carrier's hardware. I can't see that much difficulty in, say, implementing a list of phrases or URLs which result in the message being dropped, given carrier cooperation. (The carrier cooperates or else they end up called into Congress over and over to testify about such-and-such)

Standards don't provide such filtering functionality and telcos operate their equipment as a black box, so they don't have ability to implement additional features, especially when there is no business case for that and it could open them to new liability (undermining safe harbor arrangement).