Actually, that's a generalized problem with firing protections of any kind; the harder you make it to fire anyone, the less likely they are to be hired in the first place. Another case where second-order effects can end up dominating the end results. "At-will employment" sounds like a worker disaster if you only consider first-order effects, but in my opinion it is actually the preferable scenario for a worker if you consider everything.
These second order effects are a big problem in Europe. The governments there make it very expensive to fire anyone; typically around 6 months wages for anyone laid off, I think. As a result, companies are loath to hire new workers, even when an economic upswing is under way. Result: chronically high unemployment rates and young people who can't get jobs and don't understand why, not to mention increased risk aversion for companies.
Well, for a good worker anyhow.