maybe they like to sit in tower all night to record 200

Actually I have done that quite a lot of times. Well, not really recording 200, but the chat in total to analyze the bombers landing messages.
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One of the main causes for the perception of "tower sitters ruin 'our'" ENY is the occasional observation that with high and very high ENY the percentage of tower sitters can be much higher at times.
This however is often also a
result of ENY, players sitting in tower because they "can't fly anything" (complains like that usually start at ENY 5). And this is happening much more at offpeak, because due to overall smaller numbers ENY imbalances generally tend to be more radical.
Also during offpeak, the number of 'tower sitters' can also be increased because there's often actually little to do for them when there's only very few players populating a large map.
It's very similar to the old assumption of "sideswitchers looking for war perks" are creating a huge numerical advantage (and possibly ENY issues) for for the team winning the war. Actually it was (and still is) the other way around: The impending victory is a result of that numerical advantage which has almost entirely different causes.