This is public information if you apply for an insurance policy for an equine facility. So, yes, it often is part of the insurance policy. The rates are much lower with clauses like this and the legal and financial protection for the business owner is more robust.
All of your mentioned above is also not mutually exclusive. Insurance policy stipulations can coexist with the “want more money” & “lack of trust”.
Most barns that make money “want more income from jumping lessons” and generally speaking it is good for business to “not trust clients enough”. As the clients who are actually competent jumping on their own successfully, reasonably, and without injury to horse or rider likely are not ‘high calorie’ clients in the trainer’s book of biz (as the competent alone jumping ammy may not pay for lessons often, may not involve trainer in future horse purchases - so no commission $$, and may or may not pay for show training & services). The client that is more reliant on the trainer for support, guidance, supervision, and instruction is more profitable.
As a CPA/ former economics analyst, I say it is in most trainers’ best interest legally and financially to have this rule or some variation thereof (say 18+ and nothing over 2ft).
A well run facility and biz will have this. I find places without this rule to be a big red flag of poor business and logic choices – but they often are much cheaper!