If they are willing to fight, bleed, and possibly die for this country, they are exactly the type of people we need as citizens.
You are naive if you believe they are all doing it for the love of this country. Of course they are not - it would be unrealistic to expect foreigners to love this country so much they will die for us. Too many of them have already been exposed as enlisting for far less noble reasons. I recommend a series either on History Channel or Discovery Channel that discussed the problem with gangs in our military - many of them foreigners.
More likely they join to infiltrate the military, for financial or other personal gain, to get experience for the cartels, etc. Having close family members in the military, I am very concerned about undesirables enlisting.
Above all, US citizenship should never be for sale.