Because the campaign people they're stubborn, very aggressive, and arrogant. Note that these calls aren't really from the -candidate- but rather his/her campaign staff The staff figures a few offended voters are outweighed by people they do reach.
I don't know the laws and state laws vary from state to state.
Unfortunately, even if what you question is indeed against the law, it doesn't matter because:1) A political campaign is a temporary activity. Once the election is over the campaign shuts down. So if they did something wrong they can say "well, we're done so we won't do it again" and go ahead and do it again next time. As to future elections, they're too far away to worry about. So if they block their caller-id or keep calling you (or call restricted places like cell phones or nursing homes), too bad.
In a prior campaign, I got 111-111-1111 as the number. Someone said that was a VIOP source which wasn't sending ANI as yet. I don't think that's the canddiate's fault.
I've been telesolicited by private firms claiming to have done business with me in the past. I know that was B/S since it was stuff I couldn't have used.2) Tele-solicitors who violate the law know they can get away with it because 99.999% of recipients won't bother with the bureacracy to file and keep on top of a complaint. Even if one persistent person pushes it through the fine will be relatively modest and they write it off as a cost of business. 3) All the candidates use this process, so singling one out as an abuser won't do any good. 4) I was getting flooded with calls from generic unions but the actual specific source wasn't clear. I think that's illegal but too bad for me. 5) I plan to send a certified letter of complaint to the state party HQ. I doubt it will do any good, but I'm curious as to the response they give me.