Per diem is not required at all; an employer NEVER has to provide per diem unless there is a contract (including a union agreement) requiring it, or unless it is a government/civil service job, the rules for which require a per diem payment). And if a  per diem is paid, the employer is free to determine what it will be--it can be as much or little as the employer wants. What other companies (or government agencies) may happen to pay for per diem in your area or industry is irrelevant; you get what they want to give you.