Is it legal to offer paid health insurance benefits for current employees, but not for new employees?

Yes.

It is a myth that if an employer offers health insurance benefits to any employees, it must be offered to all. It is legal to offer it only to certain classes of employees, based on non-discriminatory factors; management/non-management, office/shop; exempt/non-exempt and so on. In this instance it would appear to be employess hired before/after such and such a date.

In these instances, it is almost always that an employer is planning to discontinue health insurance benefits (which he can legally do - only Hawaii legally requires an employer to offer health insurance and even in Hawaii only some employers have such a requirement) but has "grandfathered" those employees who were already covered before the decision was made.