The Ninth Amendment of the Bill of Rights was put into the United States Constitution on September 5, 1789 and was voted for by 9 out of 12 states on December 15, 1791.
An NHL referee makes anywhere between $110000 and $255000 a year. A linesman on the other hand makes $72000 up to $162000 a year. Hope this helped.
Therefore, soccer refereeing in the USA is strictly a part-time job. However, the MLS does employ four referees full time for $40,000 to $60,000 per year in addition to what they make per game. Still a lot less than NFL, NBA, NHL, or MLB referees/umpires.