The system operator then purchases the cheapest electricity available (taking into account demand is different throughout the network, and the network can't transfer all electricity to all parts of the grid because of line constraints).
This is the cost of the 'raw materials' of the retail electricity price. A solar panel on your house really isn't any different that a little power station. You still have to transport that electricity somewhere, maintain those lines, maintain the voltage, run a power station when it gets dark, bill you, etc.
It's not a big conspiracy, it's just that the cost of energy makes up about half the costs of the electricity system. If you set at a rate higher than the wholesale rate, then there must be other reasons to do so rather than pure financials.
A good example would be if an Energy Utility wanted to attract solar customers, sell them solar systems, and put it on the bill to make them more sticky customers.