Offsides?

Why do we (Americans) say 'offsides' with an 's', when it is actually offside?

 FIFA Laws of the Game - Law 11 Offside: 

Offside position It is not an offence in itself to be in an offside position. A player is in an offside position if:  he is nearer to his opponents’ goal line than both the ball and the second-last opponent

A player is not in an offside position if:  he is in his own half of the field of play or he is level with the second-last opponent or he is level with the last two opponents.

Offence:   A player in an offside position is only penalised if, at the moment the ball touches or is played by one of his team, he is, in the opinion of the referee, involved in active play by interfering with play or interfering with an opponent or gaining an advantage by being in that position.

No offence:  There is no offside offence if a player receives the ball directly from a goal kick a throw-in a corner kick Infringements and sanctions In the event of an offside offence, the referee awards an indirect free kick to the opposing team to be taken from the place where the infringement occurred (see Law 13 – Position of free kick).
http://www.fifa.com/mm/document/affederation/generic/81/42/36/lawsofthegame_2011_12_en.pdf

See - no 's' here.

It likely is because nearly all of us grow up with American football where the term, 'offsides' - plural is commonly used.

But, even the NFL rulebook states:


NFL RULES Section 20 Offside 
OFFSIDE
A player is Offside when any part of his body or his person is in the neutral zone, or is beyond the free kick line,
or fair catch kick line, when the ball is put in play.
 Exceptions: The snapper may be in the neutral zone provided he is not beyond the line (3-18-2).
  The holder of a placekick for a free kick may be beyond the free kick line (6-1-3-b-1).
  The holder of a fair catch kick may be beyond the fair catch kick line (11-4-3).
  The kicker may be beyond the line, but his kicking foot may not be (6-1-3-b-2)
http://static.nfl.com/static/content/public/image/rulebook/pdfs/6_Rule3_Definitions.pdf

No s!

Is it like people from some parts of the country who say 'warsh' instead of wash or 'idear' instead of idea?  Maybe, but nearly all of us do it.  I rarely if ever hear anyone say offside, but rather the pural 'offsides'.  Even referees who should be students of the game or at least it rules use the plural.

I don't know...I said offsides - offside plural my whole life, but once I started following soccer throughout the world, I realized that no one else does and I changed.  Now it drives me nuts when I hear everyone say it with an s.  Even my spellchecker as I type this flags offsides with an s as a misspelled word!  I tell everyone I know how to say it correctly, but no one seems to care.  Why do I care?  I don't know.  Am I just a nutcase?

Perhaps...

I will be the guy on the sideline with the twitchy eye and nervous tick when all of the rest of you yell "OFFSIDES!".

Maybe I'll just hang out in Mexico where they say "Fuera de Lugar!"