Electronics assembly and zoning

It may date from eras in which fluorocarbons or chlorocarbons were used for cleaning? Also Lead on boards? I don't know what they use now.

Making PCBs has definite heavy metal impacts on wastewater (etching and plating baths, cleaning wastewater, etc.) unless steps are taken to mitigate them.

Mere assembly could still have an impact from solder waste, board-cleaning residues, etc. Or, they may be more concerned with direct spillage of cleaning chemicals commonly used rather than heavy metals. Or both.

Roll back the clock to the bad old days of wave soldering and using carbon tetrachloride to wash off the flux. Much of that ended up going down the drain, or out the back door and onto the ground.

Here's a local example. Watkins Johnson in Scotts Valley CA had a small assembly shop running for many years that just dumped waste water and chemicals onto the ground. It was bad enough to become a Superfund toxic cleanup site:

What Jeff and everyone else said. If you really want to start up an operation there, and if it's going to employ enough people and pay enough property taxes, you may be able to show the folks in charge how things have changed and how you'd mitigate the risks and get either a zoning change or a variance.

That would probably depend on what sort of PCB-washing process is (or are) used on the site.

It would not surprise me if this particular regulation dates back to an era when solvents such as TCE were quite commonly used in electronics manufacture and assembly. In my area (Silicon Valley) they're still finding, and having to remediate plumes of TCE and similar organic solvents which were flushed down sewers, or stored in tanks which leaked, and which have gotten down into the ground-water. This has resulted in TCE contamination of water wells, and in TCE vapors seeping up into homes built in or around the old industrial areas.

A "green" electronics-assembly operation might be able to get a variance - e.g. if it did not require or perform PCB cleaning post-assembly, or if it used some sort of closed-cycle water-wash cleaning which had its own water repurification system, and an approved "haul off-site to a certified waste handler" process for dealing with residue material from the wash. If you don't have to discharge any post-wash liquids into the drains, and don't have to store or any solvents on-site, you could make a good case that there's no more risk from this operation than from a normal office-type light industrial operation.

Use of lead solder may be an issue as well.

I wouldn't put it past the realm of possibility that someone saw that they deal with PCBs (printed circuit boards) and banned them because of toxicity of PCBs (Polychlorinated biphenyls).