I'm sure everyone will yell "hip hip, horray!" to this, but it's bad for reasons that aren't obvious. When you have a financial network which has more or less a monopoly on electronic transactions making decisions about who can and cannot make transactions based on arbitrary criterion, the door is opened wide for abuse.
Your argument goes wrong for a variety of reasons. First, it's based on an incorrect premise. Visa and Mastercard don't have a "more or less monopoly" on electronic transactions. You mention Paypal, which is an alternative, and there are a number of other ways to pay online, like Amex, Dwolla, Bitcoin, PayByCash, Noca, WorldPay, etc. Visa and Mastercard may currently have the bulk of transactions, but I suspect that would change pretty fast if they started cutting off payment access for political speech or other groups that take donations, as opposed to illegal activity.
Second, while your point about Wikileaks is an important one, but it’s not as cut and dried as you make it sound. Wikileaks does indeed encourage illegal activity (as opposed to a PAC), which is against Visa’s EULA. But that’s partly just the justification they needed to use. If a series of three-letter agencies come knocking on your company’s door “asking” you to stop doing business with someone they've branded a criminal, most people won't blame you if you do as “asked”.
If you don't like the fact that leaking state secrets is illegal, then complain to your government, not the businesses that the govt pressures to shut ‘em down.
However, while Visa can hide behind legality in the case of fraudsters, scammers, pedophiles, and leakers, they can't use that same argument to stop payments to racists, sexists, or communists, because none of those things are illegal in the US. There could be issues in places like France and Germany, where certain speech is illegal, but you have to respect the wishes of the people that choose to outlaw such things.
I don't see a slippery slope here, I see the slightly fuzzy lines that exist in every international business situation between legal and illegal activity.
Finally, comparing a business decision like this to vigilantism is ridiculous. If I own a store and I don't want to do business with the neighborhood drug dealer, I’m not being a vigilante; I'm exercising my freedom of association. Visa isn't hunting down scammers and breaking their fingers so they can’t type. They are simply refusing to do business with them as any company has the right to do.