Despite excellent marketing to the contrary, the US military is not volunteer. It is professional.
When signing the papers you agree to a term of service with AND in the fine print it says the government can extend this term unilaterally. Now, you could easily convince me that any private company getting an 18 year old who received straight Ds through four years of HS English to sign such a contract would lose handily in court on the grounds the contract is abusive, but that obviously isn't going to happen with the military.
Of course, an E1 is in the 87th percentile of yearly income globally. Yes, I put my asbestos suit on before posting.