Answer:
From a legal point of view this policy and the contracts that it includes are legal since the students are adults and can legally sign voluntary agreements.
Even though this policy is legal, I really doubt that it is ethical. My biggest concerns are not with the firm, but rather with the students that receive money in exchange for future employment. The accounting firm is well respected, so $2,000 will not be a lot of money for them even if they hand them out to several students. Students also probably need the money, but as accountants they will need to make sensitive judgements that will affect their clients and many stakeholders. Remember that this bonus is given to students that haven't yet decided what to study, and if they accept it and become accountants, their only motivation will be the money they can earn.
That is where our problem lies, is the company being irresponsible for hiring people whose main motivation is solely making money? As CPAs, that might be very complex and dangerous in the future.