Partially it is a scale thing. The state only has so many inspectors, so if you limit the law to large industrial users you don't need to have inspectors driving around to tens of thousands of tiny single-use boilers inspecting them for environmental requirements they would never be able to afford in the first place.

Then it seems like scale should be part qualifying for the exemption. In this case the power plant is absolutely at a large enough scale that it should be regulated.

> Why should those private plants get an exception?

Ask the law makers who wrote it.

> All you jave done is provide a "who", not a "why".

The word "private" is the "why".

If you want an analogy: Do you follow food safety and preparation laws or submit to inspection in your private kitchen before making yourself breakfast? No. Because it's private.

You analogy is completely inaccurate and flawed. "Private" commercial kitchens (e.g. on campuses) absolutely have to follow food safety regulations.