Working with a prospective new client. They are expanding their business to include a location on tribal property. They have been advised by their current TPA that the employees of the tribal location cannot be included in their existing plan. I am aware of various special rules for plans offered by tribal governments, but I've never heard of not being able to include employees of a private employer in a plan just because the work site happens to be on tribal property. Where can I find more information about this?

How about where the TPA gets its authority? I am sure that they will be pleased to display and share their erudition, or you will be pleased to see them fall on their faces.

I'm not positive it is the fault of the TPA -- this client seems to have selective hearing and memory.

I agree with QDROphile. But as a further suggestion, could it have something to do with the location (tribal nation) not being a legal part of US?

The job postings for the Cherokee casino in Tulsa clearly show they offer a 401(k). The casino is 100% on tribal land. I found an article that says Federal Circuit Courts have ruled differently on whether ERISA applies to tribes. But I'm baffled unless as leevena suggests it's a misunderstanding of tribal sovereignty or a result of what district the land is in.

You could try the tribe itself (surely your client isn't the first company the tribe's done business with) ("and don't call me Shirley"). No idea if the Bureau of Indian Affairs would be of any help on this.