Being a Research Assistant (RA) or a Teaching Assistant (TA) is not a job per se but part of ones training while in grad school. Whatever you're paid while performing such duties is just a stipend, and not a salary in the traditional sense.

So, no, universities are not going to hire Americans to RA and TA positions if they have no incentive to be in grad school.

Foreign students come here because they want to come here to learn. Being paid for RA and TA responsibilities is just a bonus. Professors reap the benefit of having some of the best minds from around the globe around them.