They don't have an obvious foreign power to which they may have pre-existing relationships. They could still be motivated for financial, ego, or ideological reasons.

USSR/Russia was more often ideologically motivated people (and very small amounts of money). Israeli spying in the US has been "for the benefit of Israel" (and possibly tacitly supported by some of all of USG, potentially somewhat officially). Chinese spying seems to be largely of the "loyalty" form. A lot of it is pure commercial or military technology from commercial entities.

All things being equal, someone with minimal foreign relations is going to be a better security risk for USG than someone with extensive relationships with foreign entities. For intelligence aimed at a foreign country, it's obviously an asset to have a lot of interconnectedness with that country, so it's a balancing act; you'd probably be willing to take that risk for a case officer but probably less so for a support staff. For positions where technical skill is required and in short supply, it may be worth that risk too; you'd have to look at the totality of the situation.

Since China has such an active intelligence program focused on the US, it's a greater risk than, say, Germany or the UK.

but a naturalized citizen is more likely to be committed to the ideals of the new country? Why else would you go through that (especially given that countries like China don't allow for dual citizenship)

By Native Americans do you mean indigenous Americans (i.e. American Indians), or just people born in the USA in general?

Native Americans is always understood to mean the indigenous people of North and South America and their descendants. "American Indians" is a term that has fallen out of favor due to its roots in colonialism.

Edit: Yes, technically, people born in North and South America now are native to it. But Native Americans isn't a technical term, it is a cultural one that is widely understood to be so.