The 1% you are referring to is a legal limit (at least in EU), exceeding that means your use will be illegal and your local authorities might fine you.
Why limits for each device and allowing multiple devices? Probably because it is much harder to write rules when multiple devices are involved. However I did not write the rules so I don’t know the reasoning.
Keep in mind LoRaWAN can not scale if everyone starts using 1%, even if it is legally allowed. With a radio that can be heard 15km away and might well cause interference 15km away you can’t behave the same as with for instance WiFi which can’t be received 100m away (with normal antennas).
Because LoRaWAN (in EU) has no listen before talk everyone using 1% would result in massive packet loss well before there are 100 nodes on that channel (using the same spreading factor). Where a well designed network with friendly users scales to multiple thousands of nodes for a gateway, when looking at the limits (1%) you will run into issues at (guestimate) 400 nodes transmitting 1% of the time. And at that time other users of the same frequencies will also suffer (as in be unable to use those frequencies).