Why do incoming spam SMS cost money in the US?

I'm originally from Europe, where strong consumer protection laws make it free to receive SMS.

Here in the US I activated a prepaid plan, which charges $0.20 per SMS, including incoming ones.

The problem is that the phone number I got must have been used in the past, because I receive tons of confirmation SMS and marketing spam, which all are charged against my plan.

How is this legal? I purchase $10.00 of credit and it gets spent on nothing I personally did, with no way for me to block it.

This looks really punitive to the people who cannot afford the $19.00 option for unlimited text!

Are there ways to deal with this situation? If not, how can I help the US society to change this practice?