Americans seem generally to be far more patriotic than their British counterparts and their citizens have a much greater sense of national identity. 911 hurt them deeply and, presumably, they see their servicemen as fighting to 'show the perpetrators who'se boss' for want of a better experession.
We on the other hand appear completely confused about who we are, what we represent, and what is right and wrong. We are engaged in conflict with people who many of our countrymen see as brothers and allies. As far as many are concerned it is the USA who is the enemy, not the arab states we are currently deployed amongst. Furthermore, the forces are seen as part of the 'establishment' which many people despise.
Add all of those things together and its hardly surprising that when one goes around town in uniform one is met, not with approving looks, but scowls, muttered oaths and even direct hostility.