"Being illegally present in the U.S. has always been a civil, not criminal, violation of the INA[Immigration and Naturalization Act]"
"Criminal violations of the INA, on the other hand, include felonies and misdemeanors and are prosecuted in federal district courts. These types of violations include the bringing in and harboring of certain undocumented aliens, aliens (INA §275), ..."
As can already be deduced from the above, illegal entry is a misdemeanor. Only the bringing in, harbouring, and certain specific aggravation conditions raise it to a felony.
Also misdemeanors are still considered serious crimes. I don't get where this hand-waving "because it's just a misdemeanor" comes from.