You mentioned the number one reason to keep it hidden, especially for the rear: kids. Kids would be attracted to a red handle that was easy to pull and they don't care what is written on it.
As a side note, the old Model Y manual mentions on page 15 that under the door pockets there is a manual release tab for the rear doors.
https://tesla-info.com/doc/model_y_owners_manual_north_america_en.pdf
This is gone from the latest manual, so not sure if they removed it in the 2021 models:
https://www.tesla.com/sites/default/files/model_y_owners_manual_north_america_en.pdf
Also not in the html manual (which looks new? I just found it on google, didn't know a html manual existed):
Model Y Owner's Manual | Tesla
Sure, regulators can force it, but given there are a bunch of vehicle types where it's difficult, if not impossible, to exit the rear seats in an emergency (without somehow reaching the front), regulators obviously have not seen it as something to address or mandate.
For cars where kids are not a concern, the reason is likely to prevent possible damage to the door trim when using the emergency release. Many of the cars using electronic releases have frameless windows (one of primary reasons for using an electronic release in the first place) and the manual release will not drop the window down to clear the trim. So you have to be extra careful when using the manual release. The Corvette C6 is no exception:
Emergency/Trunk Door Release Doesn't Drop Window - CorvetteForum - Chevrolet Corvette Forum Discussion
This had been a common complaint of the Model 3 front manual releases actually, that it's too easy for people to pull (many times the first thing they intuitively want to pull given they can't figure out the button). Of course, Tesla have long updated the software to use the door ajar sensor to drop the windows, but there is still some delay involved. An electronic latch ensures the window always drops before the door opens.
Designers have a bunch of different concerns to balance before they consider emergencies. For most people. keeping a emergency window smasher or two in the car probably would be much more useful than stressing out over the rear doors, as plenty of accidents would have the doors deformed such that you can't open them anyways regardless of a manual latch.