It's scary because let's say you are competing with someone, so you decide to copy their API to make it easy for people to migrate over to your system rather than use their system. Now, instead of just doing that, you have to get a license to use the API. Otherwise, you are a copyright infringer.

Take this as an example: https://docs.us-phoenix-1.oraclecloud.com/Content/Object/Tas...

Here is an example of a potential copyright infringer copying the API for another competing service. I'd hope they acquired a license for it, otherwise.