Virtually every state in the country is at-will, which is a good thing. But the norms of this industry put the onus on the employer to (a) verify that candidates can execute before extending offers to them and (b) demonstrate conclusively that the fault is with the employee before terminating.
A trial period essentially tells candidates "your first three months on this job are a kind of extended interview". People are rejected after interviews based on standards of evidence far, far lower than the ones commonly used to terminate employees.
I'm fine with employers hiring in good faith, and then firing within a few months if the candidate isn't a good fit. It's far more efficient and easier for everyone that way, instead of dragging down the team, creating a burden on others, etc.
A trial period essentially tells candidates "your first three months on this job are a kind of extended interview". People are rejected after interviews based on standards of evidence far, far lower than the ones commonly used to terminate employees.