They could build a cross platform tool that worked the same way as Apple’s Find My network. They instead chose to create a tracking network that does nothing to preserve privacy.
They have every right to charge for their network if they choose. However, given their lack of privacy preservation and that (at least in the past), they reserved the right to resell that data, why would I want to opt-in to it?
No one is forced to opt-in to the Find My network. While its functionality has been expanding, it has been around for years. Given that protecting Apple Devices from loss and theft is at the core of this service, it makes sense that it should be free. Tile could absolutely charge for their service, if they offered something that Apple did/could not, for example cross platform support.
No one is forced to opt-in to the Find My network. Everyone who does so explicitly has to opt-in. Are you arguing that Apple should now be forced to charge for a service they have been providing for free so that Tile can now charge for a service they used to provide for free?
No, it is not opt-in for ad tracking, it is opt-in for third party app tracking. First parties can track to their hearts content. Second, no one is automatically enrolled, when setting up a device, users are required to consent to location services if they wish to take advantage of the service. Users may choose to participate in the service or not.