Not knowing the exact facts, so some here is speculation ...

Any private industry served by their own tracks has a reasonable expectation that the serving railroad will switch them. The industry also has the option (assuming they have the neccessary track arrangement) to provide thier own switching - either directly or through a third party provider. Class 1s used to give a discount for industries that did their own spot and pulls so all the Class 1 had to do was pick up and set out on a yard track.

Note that the 4,000 acre industrial park is owned by the City of Kingman. The tenants of the industrial park may have been served by BNSF - by over a switching lead likely owned by Kingman. Does BNSF pay Kingman for use of their tracks to access the customers? Kingman has simply contracted a company to provide switching services over and located on Kingman's track.

The tariff issue is a problem. I doubt there is a requirement for BNSF to offer a joint rate/reciprocal switch or revenue sharing with a third party provider. This does mean that the customers may have an increased cost, which could in turn make BNSF a less competitive option. Then again, this might require the rail switching provider to maintain a higher level of service (and maintain the track at their expense). If a car is spotted wrong, most customers have to wait a day or more to get it respotted.

If this is a government contract - why don't prevailing wage rules apply?

This is not an uncommon situation. There are many private or publically owned industrial parks that are in a similiar situation. The union's contract and rights only extend to the railroad they are working on - not another owner's track that happen to operate over.