They can legally fire you on the spot, yes, but all that means is that the fired person can't sue their former employer. That doesn't mean it's actually a good

They can legally fire you on the spot, yes, but all that means is that the fired person can't sue their former employer. That doesn't mean it's actually a good strategy for the employer to do so frequently, because the employer's relationship with any given employee doesn't exist in a vacuum, and has effects on the employer's relationship with other employees and potential hires.