Well, rougy, any ideology that has a 'monopoly' is mandatory, same goes for the Social Contract idea where people 'agree' to pay taxes in order to get a safety net of welfare.
You're right that it's not really a choice if there is only one choice.

I hope you'll agree though, that there is an aspect of voluntariness in Capitalism, in that the individuals choose between competing providers of goods & services, or even choose to set up shop themselves.

Of course, I'm not defending the actions of the wall street people who (among others) caused the crisis, in my opinion they should not have been bailed out... I can see the politicians' reluctance to do so though, since a big market correction is not a pretty thing. The crisis is being used to attack the foundations of Capitalism though, which I don't think were at fault.
Like Michael Lewis explains, they basically forgot that they cooked the books (and ratings agencies and so on), and fell for their own con. That's stupidity, which is a danger in any system.

And Netrunner: Capitalists can feel morally outraged about taxation, because no matter how little they worked for the money, taking it by coercion is more despicable still, and in addition leads to ill effects like corrupt politicians and counterproductive incentives.

Also, I may be over-analyzing here, but when you say: "...convincing them that they got something of equal value out of the transaction." i actually disagree slightly.
If i have $15 in my pocket, and the seller has a CD that i like, i make an exchange because, to me, the CD is worth MORE than $15. Otherwise I'd keep my money instead. The possibility of mutually benefiting from a transaction is something you don't often hear about from the liberal side of the debate, to put it mildly.