If a community's usage is say 60% of what a plant can produce, and $TECH_GIANT comes and buys the other 40%, what happens next?

Power prices will go up, since any growth will exceed the available power generation capacity. Power will become dirtier, since diesel generators will fill in when there is high demand. Then the community will spend a huge amount of money building out more capacity.

I think we will all soon see, communities sharing power infrastructure with massive-demand corporate customers will end badly for the communities.