Currently, that is the only way to implement it. Some have suggested "local area" ENY, which would work better, but would be very complex to implement and what would stop someone who is ENY limited at one field to the east of an enemy field, but his field to the west of the enemy field is not ENY limited ... he and many other could still take the 5 ENY rides and pummel the same field.
The game calculates enemy/friendly per sector already as illustrated by the 'DAR-Bars'. Maybe the ENY could be mapped to fields in a sector based on the DAR ratio?...and one field radius? Overlapping ENY decrapencied could be averaged. I think the idea of ENY is valid, but it's one size fits all implementation might be creating as many problems as it solves. Something like this might improve it.
Perhaps the HTC team could comment on how difficult it is to implement, or whether it's even an idea worth trying.