It will take a large amount of people to reduce the speed of the network, unless the small amount of people all wind up trying to access the same tower during the protest.
As far as the legality, I think this is an issue that should be resolved on intent. If the intent is to conduct a DDoS attack for the sole purpose of causing economic hardship on AT&T then it is illegal. However, if the intent is to protest inadequate service issues and bring to the company's attention the flaw in its system, then I would not consider it illegal. This is why we have judges, to interpret the stickier points of law - but unfortunately there has been an active campaign to call them "activists". In other words, we don't want judges to judge in much the same way we no longer allow teachers to teach. (They are suppose to regurgitate what is before them mindlessly.)