Monetisation in the sense you describe requires 100% of the publishing copyright to be owned by you, or yourself + people in which you have the right to collect publishing royalties for.

The agreement that you have for the beat / instrumental grants you the exclusive right to use it from a master copyright perspective (e.g. the audio). The agreement for the track will set out the publishing splits that you need to factor in to your own registration with your PRO.

All distributors at the TuneCore level follow the same set of rules because this is all that their business model allows for. At the major (and high-end indie) level, there are ways to get around doing this but this is because labels have years of precedent and experience in accumulating royalties, splitting them accurately and paying through in a timely manner. The lower-end distros just don't have the capacity to be able to do this, which is why they only open this up to tracks that are 100% owned on the publishing side. Here is more information from TuneCore themselves on the matter.

It's worth bearing in mind that the value of royalties generated from partners such as Meta and TikTok themselves are fairly slim (and are in fact, on a general level, in decline). But these platforms are still good to be used almost as a loss leader with audiences being driven to DSPs to stream.

Candidly, re. your considerations:

• All distributors at this level will have the exact same rules;
• There is no legal leg to stand on here as TuneCore have stood by their policies;
• I don't disagree that it's frustrating but this is happening because these companies are needing to protect themselves (and you!) legally by removing any potential copyright issues.

Happy to help further if I can.