What purpose does the highly elliptical orbits provide that a more circular orbit doesn't? Does it allow higher resolution when it swings in closer, and that the higher orbit is over parts of the globe of little interest?
I think it's usually the opposite. The satellite spends much more time observing interesting stuff in the higher part of the orbit and much less time zipping by uninteresting stuff in the lower part.
that does make sense. US targets of interests do tend to be northern hemisphere oriented.
Look at where the sats dwell, mostly over the North Pole. Now think about the great circle routes between the USA and the USSR.
Probably higher resolution over more desirable areas as well as more stable orbits.