OP, are the per diem rates billed to the clients? If they are, and clients are billed the per diem rates, then it sounds to me like the company is hoping to pocket the difference between what employees use and what the client is billed, rather than allowing the employees to pocket it. The lack of response about opting not to claim per diem makes it sound like that's what they're doing.

I too suggest starting to look elsewhere, but also asking for a written policy, including how dietary restrictions are handled as well as if one can opt out of taking the per diem amount.