The value of a single questioner at a nationally televised event is quite high so it makes sense cost-wise to spend a lot of effort to plant a questioner versus running an ad. Hillary was recently caught planting a questioner on a very benign subject ("what would you do about global warming?") but that was just a defensive action. When all the questioners at an event are going to be someone's plants at least some of them might as well be yours.

The same thing goes double for TV ads under campaign finance reform. The federal government spends three trillion dollars a year (a slight exageration: it was 2.9 trillion last year). You'd better bet your last dollar that buying Senators will happen no matter what the law. There is simply too much upside at stake. Making it illegal to do certain kinds of vote buying just shifts the money around. At base when it is profitable to spend $1.00 to get $1.01 in return people will do it. To take the money out of the system you need to ... take the money out of the system. Curtailing free speech doesn't remove the temptation it just makes it easier for the people with the best lawyers (who are rarely the people with the general interest in mind) to speak "legally."