The various SIT tones seem to be used, theoretically, to provide some sort of in-band signaling, and as far as I know this is how Asterisk's Zapateller and the "TeleZapper" appliance worked. The Wikipedia article on TeleZapper describes the scenario though - telemarketers can simply disable the detection for such a thing, rendering it useless.
Like anything else, it may make a dent in the number of those that get through to you, it may also confuse people trying to get a hold of you. Some people have gone as far as whitelisting incoming callers, which could work as well. Anything works until people figure out how to get around it, which there seems to be plenty of money in. So, no, they're not required to respect the SIT tones but it may help, for now.
Adam P KC2YQF