Shades of Max Headroom ...
the dystopian future TV series in which it was illegal to have a TV with an off swiitch. Under what pretext would such a law be brought into being? Important national security information from your government, perhaps? After a while, what was annoying would become expected, then finally manditory.
After a few years, an argument could be made that some large proportion of the population, patriotic citizens, are accepting the messages without complaint, and therefore why shouldn't you (In Max Headroom, the important messages were advertising, critical to the national interest, but that detail was topical social commentary.)?
Hence, in this real world case, the requirement for the target to have to 'opt out', to actively resist the intentions of the security state.
This goofy idea likely won't fly, but it won't be because of the charge to the citizen for the text message.