People are wired weirdly. If they'd not offered this feature at all, people would be content. Now that they're offering it but for money, people are super upset.
I do agree it's a somewhat weird thing to do: they're very likely not going to make bank with a feature that very few people would want (out of those who even bought the a7IV) and even fewer of whom would pay for it. So why make it something you pay for at all? The negative reaction was absolutely inevitable, whereas a "hey, here's something cool our cameras can do now" might have been a positive PR move.