Is it legal for an employer to offer one employee health insurance benefits and not the others he employs?
If it is a difference between a temp. fulltime worker and company fulltime, no. Temp workers are like ships in the night, here today and gone tomorrow.
It is legal to offer health insurance to some employees and not to some. It's not illegal to provide insurance only to some classes of employees. It's not illegal to provide insurance only to some classes of employees. Go to http://employee-benefits.lawyers.com/Employee-Benefits/Employees-Benefits-FAQ.html#12 for details.
Not now, but when Obamacare kicks in you have to insure all employees that work over a certain number of hours. I think it's 28 hours a week.
It is legal. It could be a perk for doing a good job or years of service.