OP, it is actually a very logical policy because it is simple and to the point. You are not doing
DCL a favor by booking more cabins with fewer in each. DCL would rather have 3 or 4 in each cabin than one or two. They make more money that way. They would surely have no problem selling any cabin you let go if you choose. You may see it as being rewarded for spending less, but DCL sees the bottom line and couldn't care less who pays for the cabins. They only want them booked and paid for with the maximum occupancy allowed until it reaches the lifeboat limits per area. But the Platinum "rules" are made to be clear and concise, not negotiable.