This fact makes bitcoin even more dangerous. Miners are only going to fork the network in a direction that benefits them, sometimes at the expense of everyone else and the network. This is called tragedy of the commons, and the block-size debacle is an example of that.
The only direction that benefits miners is the direction where they have the maximum potential for profit by selling the coins they earn through transaction fees.
This will inevitably be the network which most people are transacting on.
The actual incentives are a bit skewed currently because transaction fees are dwarfed by the block subsidy.
