To meet the U.S. definition of SARS requires only that you have a history of recent travel to an affected area, plus a very common set of symptoms. The result has been an acknowledged over-statement of the number of cases.
To illustrate how extreme this has become, the U.S. was listing 166 cases of "possible" SARS last week, but only 33 actually had pneumonia. Of those 33 with pneumonia, only 5 had actually been confirmed as SARS cases.
The U.S. CDC announced last week that they were looking into possible problems over-diagnosis may be causing, including the public perception that SARS was weaker in the U.S. than in the rest of the world.
The U.S. CDC mentioned at the same time that they were certain the number of U.S. cases would fall substantially once a reliable way to diagnose SARS was found.
The idea that the SARS virus would somehow be weaker in the U.S. has not been advanced by anybody working on SARS, nor is there any medical explanation as to why that would be so.
The idea that SARS patients would have better medical care in the U.S. has been prevalent in the press. However, the medical care received in Singapore, Hong Kong, and Toronto has been comparable to what would be received here.
On balance, the most likely explanation for the better than expected SARS results in the U.S. is simply that we do not really have very many cases of SARS. Let's hope it stays that way.