> PIP recipients aren't allowed to save up for anything

That's incorrect.

Some, not all, PIP recipients are also in receipt of means-tested benefits, such as Universal Credit for example, which has a £6k capital disregard, and a reduction of benefits between £6k and £16k, and no entitlement over £16k. (Also I'm sure there are PIP recipients who work and receive no other benefits)

There surely are PIP recipients who would end up with savings of over £6k if they did not spend their PIP on anything else, and Motability is a legitimate way to spend it in the eyes of the DWP.