Sad part about this is that KLM Crown Lounge 52, which you could physically access after landing at AMS (unlike US airports, you are not compelled to immediately go to immigration and customs) is one of the nicest airport lounges I have visited. I'm guessing KLM ended access to Delta arrivals when they figured out that passengers ending at AMS who arrived at 6 am but who couldn't check into a hotel until noon or later ended up in this lounge.

If you were connecting on to some other airport, you would be able to enter either Crown Lounge 52 or Crown Lounge 25 (which is beyond the passport control to enter the Schengen zone portion of AMS).