Yes they do have some rooms that require a certain number of nights.
I booked a room 6/5 and wanted to add on 6/6 but it wouldnt let me add on that night, however if I put in 6/5-6/7 or even through 6/8 it showed the room but I couldnt book just the one night.
So I booked with 1 extra night and the guy at the front desk told me I could cancel that extra night when I checked in. There is possibly a $50 penalty if I do but its worth it not to have to change hotels for that one last night.
Also last Oct we had 2 nights booked and when I called to modify the ressie for just 1 night they said they couldnt do it. There were no rooms for 1 night?
I said well I already have the room I just want to cancel one night. Long story short I could not stay just the one night so I canceled entirely and stayed at disney.