Two things to come to mind. First, if you're actually holding a waiter's tip hostage over the ability to split checks, you're a shitty person. And the kind of person that threatens to leave a bad tip if the waiter doesn't do something is the kind of person that probably tips badly to begin with.

Second, time is a zero sum game. If the waiter has to spend several minutes wrestling with the POS to split a check, that's several minutes they can't be serving other tables, which if the restaurant is busy can mean a degraded experience for other diners (and of course if this leads to someone being unhappy with the service that will likely mean a reduced tip). Of course if the restaurant isn't busy and the waiter can afford to spend several minutes on this, then nobody else has to suffer, but if that's the case I wouldn't expect the waiter to balk at splitting the check (assuming they're capable of doing so at all, which as I said before may not be true depending on the POS or on restaurant policy).