Map-restriction.

[Lantry]

Yes, the map restrictions limit creativity, but only the really bad creativity (if there is such a thing). For example, when I started playing AC I liked maps like poolcarnage. for those of you who aren't familiar with this map, it is basically a swimming pool. Not exactly official map material, but still very fun and playable. What people don't understand is that restriction isn't the difference between "any maps you want" and "only official maps"; there are still a lot of maps that are very creative, yet fit perfectly within the requisites. For example, ARG coloseum and LAZERBEAM!!. LAZERBEAM!! is especially creative, as they did a neat thing with height fielding and water to make the floor look really trippy. Both of these maps fit within the requisites, yet are also very creative.
Tl;Dr version: map restrictions are not restrictions on creativity. If you take the time to download the maps i mentioned, you will see that this is true.