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.
This is true but if it's well known that you fire, say, 30% of your hires after 6 months then you'll have a much harder time attracting the kind of candidates you want to be attracting. You can compensate for the devs' opportunity cost with, well, compensation, but you'll be competing with FAANG, whose high-turn reputation isn't even 30% bad (with the possible exception of Netflix)
