1) I think you'll find that the argument that's being made, here, is a whole lot simpler. It's "If we say 'reduce the labor pool by 1 million jobs', a lot of people will think we said 'will eliminate 1 million jobs'."
Like a lot of political arguments, it's a statement that's been carefully crafted to try to make people think you said something that you carefully didn't actually say.
2) Having said that, though, yes, reducing the number of hours that people want to work can be bad, (or not), in some cases.
If some 62 year old guy decides to retire, and his job is filled by some 35 year old guy who wanted a job, but was unemployed, then the labor pool (the number of people who want to work) has gone down, but the number of jobs hasn't changed. Society, as a whole, has seen one person go from "employed" to "retired", and one person go from "unemployed" to "employed".
I would assert that that is actually a net positive, to society.
Similarly, if, say, we go from one guy working 80 hours a week, to two guys working 40 each, I'd say that's a net positive.
However, if we go from one guy working 80 hours, to one guy working 40, and his employer doesn't hire anybody else? Yeah, that was a voluntary reduction in hours worked. But I could see that as a net negative for the economy.
And, I suspect that
1) Yes, Obamacare might actually incent some people into working less.
2) And not all of them will be the "net good to society" reductions in work. (Some of them will be. Maybe even most. But certainly not all.)