Indian Tribes and Campaign Finance

I have a question. Apparently, the Abramoff guilty plea involves criminal fraud in the area of lobbying and campaign contributions from Indian tribes. Some have mentioned that the Indian gaming situation is out of control. Tribes (membership in which is often suspect) are raking in huge bucks without being subject to much, if any, regulation. The absence of meaningful oversight by states or the feds is due to the tribes' sovereignty.

My question -- foreign persons or businesses are barred from contributing to our elections. If the tribes are raking in big bucks because of their sovereignty, why are they allowed to make political contributions?

I don't see how this makes any sense. They can spend a lot of money to influence elections and lobby the government, but the government has no authority over what they are doing.