Like many things there are unexpected downsides. If a bug is discovered in these machines you cannot fix it without expensive work to get the next version audited. Often that isn't done which means if an otherwise good machine has a problem it is scrapped (or sometimes still used in hopes that nobody discovers how to exploit it - see the story linked elsewhere in this thread)

The above could include cases where someone intentionally slipped an exploit into code and it escaped audit. I'm not aware of this happening (and one of the points of audits is to prevent it), but I'm well aware of ways I can create an exploit that I wouldn't catch if I was an auditor.

Indian/Native American land is still semi controlled by the US government. While they have a lot of leeway they generally are pretty good. They are making a ton of money and they know even a strong rumor of something going on would kill them.