why should I not be allowed to support the candidate I believe in?
Nobody's suggesting you shouldn't be allowed to do that. The proposal is that you shouldn't be able to do commercial business with them, within the narrow context of the re-election business specifically. Support them in all politically-imaginable ways, and even form a corporation with them to sell cars, if you wish. We just want to point a gun at your face and say "stop giving them money." We want elections to become political instead of commercial.
Consider some so-called politician that you happen to hate. (Please don't tell me you're the one person in this country who doesn't hate anyone.) (Obama? GWB? Reed? Boener?) Now admit it: you don't really think of that person as truly political, do you? You could respect a true sincere adversary, but this guy, he's not quite that. He didn't win over his supporters by showing he knows how to make wise decisions; he used expensive media advertisements to trick a bunch of fools into supporting him, right?
If only Obama were the actual socialist that a certain media company says he is, you might actually hate him less. But he's not: the bastard is corruptly selling his DoJ to the highest bidder in a way that would horrify Marx and Engels. If only GWB were the conservative he ran as, you would hate him less, but at least your cold uncaring government would be cheap. But he wasn't: somehow the dimwit managed to commit to more spending of public funds than LBJ and FDR combined, funnelling it into contractors' pockets at everyone's expense.
Where's the political philosophy?
If only those people actually had to sink or swim on their actual political merits or lack thereof, then maybe your guys would finally crush that party, once and for all, and the country could get back on track. Or at least you'd finally get that fair fight you've always wanted but the country never really has, and then if you lost, well, that opposing philosophy isn't all bad. Even Marx's|Rand's society would have a few nice things about it, as stupid as it would be.
But instead those people buy slick ads, and the sheep in That Other Party keep falling for it, believing the ads and voting for the slickness instead of the politics. And the reason those other people aren't merely polically wrong (if only that were their failing!) is that the ads they use to buy the foolish voters are expensive, so they owe favors. Thus, their misguided conservative|liberal foolishness, goes beyond wrong, into corrupt.
Regulating the election business is one proposed solution to that, for allowing things to get back to politics and allowing democracy, instead of media ad budgets, decide our fate.