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If you don't want to contribute back to the original project, and the license doesn't force you to (few do), then you don't have to. You are not even forced to improve the software at all.
However, if you do make and distribute changes but don't try to merge them upstream, you are not doing anything wrong, but you are also not being a good citizen either. You are taking what upstream gives you, but you are only making the improvements available to your sub-community.
Expecting upstream developers to go around looking for stuff to integrate is just like saying "we take your stuff for free, but if you want our improvements you get off your ass and come get them". Doesn't sound nice, does it?