When a system is under warranty or a contractor is obligated to support/guarantee the system for X number of years they put in clauses where outside repairs void the contract. Its the risk of voiding that warranty/contract that prevents you from making repairs yourself.
For eg if its in my car guarantee book that if I get my car serviced or repaired outside the manufacturer has the right to void it's warranty.
The US should negotiate better terms with its contractors where for field level repairs it can handle inhouse without voiding supplier liabilities.
For Taliban since they will get no OEM support they can do whatever they want. Personally think alot of these will end up with blackmarket arms dealers and Taliban will use the funds generated from these for other systems they can service or use.