The theory that the power to control the Bitcoin protocol is held by those able and willing to offer things of value for bitcoins (be it goods, services or other currencies).
As long as mining is conducted for economic gain, then any change adopted by the miners needs to be supported by the economic majority for it to be successfully implemented.
While miners could adopt changes that are not welcomed by those holding bitcoins and start mining using the changes, their mined coins might not be accepted by users that refuse to adopt the changes. Thus those miners would then, in that instance, not gain economically from adopting the change. This scenario exists regardless of whether or not the miners adopting the change had 51% of the hashing strength.
So the ability for a protocol change to be successfully implemented ultimately rests with those who accept bitcoins in exchange for value. Generally those will be the merchants.
Credit goes to Meni Rosenfield for first coining the term.