How allocated ATP works with Order Management availability

We have an issue with allocated ATP where we can see that it is possible to reserve a quantity that is greater than the request date quantity.

For example we have an allocation split across 3 demand classes A - 40%, B 40% and C - 20% for item X. We have say 10 on hand with no reservations.

So on the sales order form enter item X quantity 10 for demand class A. The availability screen shows the correct request date quantity of 4, with reservable quantity 10 and QOH 10. ATP correctly schedules the order out.

However there is nothing to prevent the user going in and manually reserving all 10, thus 'stealing' the other allocations.