The Americans did deploy an ABM system in the '60s and the early '70s. The politicians decided to kill it off.
The truth is that decoys don't work and only serve to reduce throw-weight. Part of the problem in ABM debates is that the technology that makes them work, such as decoy discrimination technology, is highly secret because it is used in other defence areas, so we only hear one side.
Btw the MARV doesn't make the sort of evasive maneuver a bomber does and it isn't really much of a problem for the defence anyway, as I understand it. There is a good discussion on the various types of RV and their effects on an ABM system here:
http://www.tboverse.us/HPCAFORUM/phpBB3/viewtopic.php?f=11&t=70.
If you do want to make life difficult for the defence build a fleet of bombers in the class of the B-70.