The TCPA (Telephone Consumer Protection Act) is the US federal law that requires prior express consent for automated marketing texts, with statutory damages per message for violations.
The TCPA, with the FCC rules under it, is why US marketing SMS is a consent-first business: prior express written consent for promotional texts, honoured opt-outs, and per-message damages that class actions have made very expensive. It applies to the sender, wherever the sender is, for messages to US numbers.
It is one of the reasons US traffic is registered and filtered so heavily by carriers, and why a non-US sender should treat US marketing as a compliance project rather than a destination in a list.
At SMSMeteor: sending to the US is possible from numeric senders; the acceptable use policy requires consent for marketing, and the legal guide explains what falls on the sender.