So let's look at what a coach is . . .
According to Merriam-Webster, a coach is:
one who instructs or trains <an acting coach>; especially: one who instructs players in the fundamentals of a competitive sport and directs team strategy <a football coach>
Now, if a coach is "someone who instructs or trains", should we assume the coach possesses some knowledge of the game? If we were discussing competitive sport, then yes, the coach more than likely has an advanced knowledge of the game. But we're talking about house league sports, where coaches are always volunteers (coaches aren't "appointed" or "hired", as they would be for competitive teams).
As volunteers, knowledge of the sport is rarely a requirement. Many leagues are simply grateful for a warm body to lead the players in a team cheer and manage their playing time. But, this isn't always the case.
All hockey in Canada is governed by Hockey Canada, which has created the National Coaching Certification Program (NCCP). This program has components for recreational/house league, competitive and High Performance. Each provincial association, for instance the Ontario Minor Hockey Association (OMHA), administers the training and monitors progress. It's up to the local hockey leagues to ensure they comply with Hockey Canada and their provincial group in regards to who gets behind the bench. There are other requirements that we'll save for another day.
Sadly, hockey seems to be the rare sport where certification is a requirement to be behind the bench, even in house league! A quick search of soccer, baseball, lacrosse, football and dance reveal there are no prerequisites to coaching those sports. Each of these sports are adopting the NCCP, which is a requirement for competitive coaching, but are not enforcing it for house leagues.
Another sport that requires knowledge and certification of its teachers are the Martial Arts. To be recognized as a sensei or teacher, one must achieve a third-degree black belt, or sandan. You may assist the sensei as a senpei, but are not allowed to lead your own dojo until you have achieved some level of competence. (Wikipedia article)
I firmly believe that you have know the sport you are trying to coach, in order to be a successful coach. How can you teach children a sport if you, yourself, do not understand what you are trying to teach? How do you explain the dynamics and intricacies of a game if your collective knowledge has been gleaned from SportsCentre?
The answers lie somewhere between the three organizations noted above. Hockey requires a coach to know how to coach, martial arts require a coach to know how to perform, if not master, the skills and most sports need someone to step up and lead the way.
There is a place and a roll for everyone who wants to help out. Organizations just need to help people get to the point where they can provide the most help.
After all, It's Only House League Sports!