Don't you mean that IHG is just like Marriott in this case (given Marriott has no restrictions around length of stays for points accruals)? Though in total program comparisons, I would agree IHG isn't Marriott.

Starwood does have a restriction which I think is now 180 days (used to be much less.) The Marriott/Starwood merger won't be completed until after 2018, so I doubt there would be any change in length of stay requirements from either side until after the merger.