As far as I'm concerned, yes. It is absolutely is.

If the "Federal" government wants to levy taxes against non-Indian corporations, it should be done at the corporate level and not from the Indians. It's simple. The Indians have every right to become as entrepreneurial as they choose, free from any additional annoyances or interference from anyone.

The US government declared them a sovereign nation. So, I ask you... what are those boundaries?
Everyone's entitled to their own opinions, but not their own facts.