Because iPads don't require activation, and carrier services do require activation. The "Activation Required" message is specifically related to cellular networking.

This could be (and I suspect very likely is) a problem with your iPad's SIM that you could check by powering off the iPad, removing the SIM and turning it back on again. It could be that there's some kind of corruption that you might need to fix by connecting it up to iTunes or even bringing to the dreaded Genius Bar. It could conceivably be some weird bug in iOS that has somehow sprung up and turned your iPad into a flat flowerpot. All of these things suck! But what it is not is Apple forcing you to "authorize" your iPad in order to let it boot up.

Hmm. Okay, that sounds reasonable! I've come across the "Activation Required" screen before, both with people asking similar questions specifically about old cellular iPads, and it specifically says "Activation Required" while it's waiting to talk to an activation server, and that server really does cellular configuration. (I have personal experience with this step being screwed up on a new iPhone once. It was not a super fun time.)

A bit of Googling shows the "Activation Lock" screen as a screen that says, well, "Activation Lock"; I think it's really a separate thing. If that's what happened to the OP, then it can be unlocked from the web:

https://support.apple.com/en-us/HT201441