Because, regardless of what is painted on the ground, you have to recognise when the pads are being used correctly and when they are not.
If you assume they are being used correctly but don't bother to check and then exercise poor airmanship by not thinking about where your rotor is in relation to the aircraft on the ground then you have compounded the mistake made by the crew that didn't position the parked aircraft.
Just because it was parked in the wrong place doesn't absolve you as the pilot of the landing aircraft from the responsibility of keeping clear.
As you come into dispersal and see (positively) the other aircraft on its prescribed square then you know landing on the other square will keep you clear.
If you see the other aircraft isn't on its prescribed square then you must take extra care and modify your landing spot to ensure adequate clearance.
It is absolute basic airmanship.