Why? To give them the chance of beating the Third-Party quote? Or maybe offering an Extended Warranty?
Or would that be because the equipment has been leased from the OEM ... which would, of course, undoubtedly contain a clause that repairs may only be carried out by the OEM (which would be fair enough).
Otherwise, the OEM does not own the equipment, and therefore has no claim on it. In other words, the owners can do whatever they want with it (within the Law, of course).