Neo-Con foreign policy has always maintained that there should never be any other super-power except the USA itself again. As a matter of pragmatism Mogolia, the US, and China, all know that if China wanted to invade Mongolia it could. However, that could not occur today very easily with todays complicated geo-political politics, especially in light of all the trade that occurs between the US and China. In order to contain China we need to have the country surrounded either by countries controlled by US troops, or those countries that are definite allies. This keeps China on a short leash, and it is probably the wisest thing to do at this juncture, considering that China will be our biggest competitor for oil (a dwindling resource) in the next 30-50 years.
China won't come to blows with Mongolia or the US over US help of the Mongolian military, and it never hurts to have more friends.