Why not though? We are fecking shite for all the money and the name we have. Mourinho is jobless at the moment and very likely can choose from some top jobs (he already refused Inter and Roma; there are rumors that Madrid are interested). That clause would just ensure that United won't feck him (again, if some other rumors are to be believed).
If we agreed in that clause, it means that we want him. But of course, Mourinho wants to be ensured just in case that SAF sees a dream that Giggs will stay here as a successful manager for 25 years and sells that to the board, and them in infinite wisdom give Giggs a 6 year contract. Or worse, Ed decides to continue with LVG.