The miners have the actual power, but want to avoid a situation where they are directly fighting against Buterin and the developers, because that would cause a crisis and tank the value of ETH and hence their income going forward. A hard fork in this situation (developers update protocol with POS, miners keep using existing version) would start with the miners having the biggest chain by far, but the new POS chain would be followed by a large part of non-miners immediately, and grow from there over time, eventually becoming the largest chain.

So miners have to make a cost-benefit analysis as to how much control they can exercise while not causing a hard fork as well as what the consequences of causing a hard fork would be. Likely right now they're using accrued coins to prime their stake in PoS rather than directly working against it - delay helps them here.