In most jurisdictions felony murder is actually a lesser form of murder.
Generally a felony is a series crime, a misdemeanor is a petty crime. Misdemeanors usually carry a maximum punishment of less than one year, felonies have much higher punishment.
As far as what crimes are classified as what is up to the legislature, obviously, but the legal distinction is more significant than just a label. One of the biggest distinctions being that felonies require a grand jury indictment (in states that have grand juries), or a probable cause hearing in non grand jury states.
Also it's not like the U.S. invented the distinction, we inherited it from England.