Is it legal for a employer to make an employee sign up for LLC so the employer wont have to deal with taxes?

You don't sign up for LLC...it is a form of corporate entity and is formed...many others may be equally or more appropriate in most circumstances.

An employer may employ anyone they want, or dischharge anyone they want and hire anyone or thing (outsource) that work. It is legal for him to do business with whatever forms/people he wants.

However, if you are an employee or an independent contractor (which changes the taxability) is actually a facts and circumstances finding...and there are a number of easily found lists of rthe items the IRS considers. The existence of a claimed LLC vendor relationship would be seen through and not carry any weight with the Feds anyway.