- Horizontal agreements, like the one you describe, where competitors cooperate in one way or another. Apple and Steam are not cooperating here.
- “Single firm conduct,” i.e. monopoly stuff.
Which laws do you think Apple is violating here?
Personally I think mandatory app stores run afoul of exclusive dealing laws which do require a showing of market power to take force. The FTC had no problem enforcing this in the pipe fittings market [1] which has very healthy competition from china. A total monopoly is not required.
[1] https://www.ftc.gov/enforcement/cases-proceedings/101-0080b/...
Would Nintendo be in the wrong if they didn't allow PSVue on the switch?
Every curated platform controls what can be in it - including all of the console manufacturers.