I suspect that in-stock cars have already generated any manufacturer, group or dealer volume incentives for the dealer so, unlike a factory order, they will only make a handling fee (£230?) from a Motability transaction in these cases, whereas they will potentially make a few grand from selling the same car to a private customer. By placing a factory order for a Motability customer they will get a new sale which counts towards their bonus targets, so they make a lot more than just the handling fee. I can understand why some think it’s discriminatory and, in a way, it is, but they are businesses not charities, as others have said.
Some dealers will allow Motability customers to take an in-stock car where they know that they won’t shift it quickly to a cash buyer. That could be for any number of reasons – a surplus of stock, cancelled orders, unpopular colours, a need to have the car registered by a certain date etc etc.