Wait. Really? I had no idea this is true. You're saying there's a US law that means replying STOP is intercepted by verizon/T-mobile/etc. and the sender never has to see it?
This -- Stop is required action, but its passed to the vendor for management -- non-compliance is punishable by loss of shortcode/carrier acceptance.
Things may have changed since, but used to work sending 8m sms messages before breakfast every day to people who subscribed to news, sports scores, etc.