The Why..
The MEL is developed from the Master MEL published by the manufacturer and must not be less restrictive than the MMEL.
For whatever reason, your Flt Ops Engineering department developed an MEL with this restriction. As such, the CAA approved it. As the MEL is revised frequently, it may be a good idea to drop by the office and ask about it. These things do get involved and seemingly minor things. An be missed, overlooked or need a 'tweak'. In any event, the MEL that you are working with governs your operation.