I'd urge anyone who finds this behaviour unacceptable to contact the UK's CMA - the regulator responsible for competition. They have an ongoing investigation into Google and Apple's practices with regards to app stores (and mobile platforms in general) [1], and have been looking for evidence on the topic of competition in app stores, and dominance. While the UK is just one country, the CMA has a wide mandate to act o…
The FTC has the Bureau of Competition[3].
Each individual state has an Attorney General office that will hear these types of complaints, and some of them have explicit antitrust divisions[4].
[1] https://www.justice.gov/atr
[2] https://www.justice.gov/atr/report-violations
[3] https://www.ftc.gov/about-ftc/bureaus-offices/bureau-competi...