There is clear law on that issue: https://oag.ca.gov/SB657

Right now, there is no law that app stores have to be open, or that they have any certain policies. Certainly, there is appetite for that law. Another comment says that Google Music costs 30% more on iOS, but Apple Music costs the same on both platforms. That is never a good look, but it is not clear that it is illegal. A law could be passed to make it illegal, however.