It's done on purpose. Scale only accepts zero when enter is pressed, not when typed or focus lost. Zero scale can cause problems for other parts of the animation because it puts a wrinkle in the math. I don't remember the exact reasons why we require enter, but it was bad enough that we wanted to make sure the user wants that.

It is allowed in animate mode, but requires pressing enter.

It is not allowed in setup mode, instead you'll get the smallest possible scale value. This is because zero scale in setup mode gets multiplied with the scale set in animate mode, which would always be zero so you'd never be able to get any other value.

Not sad, just press enter!

True, enter is typically used to end editing and keep the current value, so focus is lost. I wish I could find the discussion that led to requiring enter for scale 0. I'd hate to bring it back only to go in circles. I think it was related to values normally being applied as you type. Say you want 1.5 so you press 1 and it shows scale 1 immediately, then you type .5 and it shows scale 1.5. But if you want 0.5 and you press 0, if we show scale 0 immediately I think the mathematical singularity caused by 0 was affecting other parts of the skeleton (constraints?) which stayed affected even after you finished typing 0.5. Assuming I remember correctly, that's an argument for not showing 0 immediately, but doesn't explain why enter is required. I can't find the discussion but I'll ask around and see if anyone remembers.

If zero value is causing problem, the program can just add auto-correct logic for places where zero is not expected to make it close-to-zero instead of absolute zero, and have the zero handled like normal value in the input field. At least if zero in animation mode is allowed, it should disable this behavior in animation mode.

It also find this zero input issue unnecessarily little annoying. It doesn't break things but it surely affect the mood.