Nothing was stopping them from doing that before. Individual health insurance plans were always available. The reason companies offer health insurance is as a benefit to make their employees happy. If they stop the benefit, they risk losing their employees to another company who still provides it. And unless their employees are below average wage, the employees are going to have to pay more for their insurance now than they would have on the individual market before Obamacare.
The “free lunch” logic applies to employers as well as individuals. If they’re think they’re going to save money by pushing costs on the government, who is only going to drive costs and inefficiency up more and eventually have to raise taxes to run a health care system more expensive than it was previously, they’re wrong.
obamacare changes the equation, because plans will be limited by what they must provide and the cost of bureaucratic compliance being so high businesses will find it cheaper to drop the benefit. Losing employees is no longer an issue. We now have a new normal of 7% - 9% unemployment. We do not have a rapidly growing economy and because of the debt and QE we won't for a very long time.
Employers have wanted to decouple healthcare from employer-provided benefits. Obamacare gives them the tool to do that. The fines for not providing it are far less than the costs of providing it to their employees. Tens of millions will be dropped from employer plans and forced into the exchanges. It is logical and human nature for most employers to make this decision. The financial incentive is clear.
Employer mandate
Beginning in 2015, employers with 50+ full-time employees or full-time equivalents must offer medical coverage that is "affordable" and provides minimum value to full-time employees and their children up to age 26 or face penalties.
Coverage is affordable if employee contributions are less than 9.5% of:
Employee's W-2 wages
Employees monthly wages (hourly rate x 130 hours per month),
OR Employer Penalties