No, Romney's record is not off-limits. No, free market capitalists are not morally bound to defend all practices that occur in those markets.
The biggest problem with the political attacks on Bain Capital, quite honestly, is that the politicians don't really understand what that business does in the first place. That's why Rick Perry makes idiotic statements about "vulture capitalism," and Newt Gingrich suddenly believes what he reads in the New York Times.
The grand truth about free markets is that they encompass all the choices of all the people, which, in sum, provide the greatest benefit to all of us in terms of growth and prosperity. That doesn't make all choices equally good - it simply protects, and benefits from, the right to make those choices. The fact that politicians can't understand particular businesses is not a fault of those politicians, but a description of all politicians; indeed, of all people.
What makes the current attacks on Bain "from the left" is not that they are "attacking free market capitalism." They're not. The problem is that they are based on the premise that the politicians can judge which businesses are most worthy. That's Obamanomics in a nutshell: picking winners and losers.
It would be legitimate, even conservative, to question whether Romney's business management background really is helpful experience for the job of president. That's an idiotic notion that started, I think, with Herbert Hoover. That's not to say somebody with extensive business experience is uniquely unqualified for the job, but the fact is that Romney's Bain existed primarily to pick winners and losers. It was the exact opposite of what the government needs to do. Turning around a failing company is not the same as balancing the Federal budget - not even close.
And the business school consensus building that often passes for leadership in private enterprise, gave the people of Massachusetts Romneycare. You'll be forgiven if you have a hard time distinguishing between that and Obamacare; Obama's folks can't tell the difference, either.
Yet Romney still defends it, and his rivals simply don't talk about it. Repeal of Obamacare is a singularly existential issue in this 2012 Presidential election year, and the current Republican front-runner takes it completely off the table.