Yes, it is legal for them to ask this of you. Remember: employment is employment at will--there is no right to a job. Rather, employers freely decide who to hire and who to not hire, and are allowed to refuse to hire someone for any reason not specifically made illegal discrimination by the law--and there is no law preventing an employer from refusing to hire someone due to concerns about a criminal background or background check results (i.e. this is not illegal discrimination). Thus, they could refuse to hire you at all; or they could legally make hiring contingent on your providing the documentation they want. If you don't want to provide the documentation they request, you can refuse to do so and seek employment elsewhere instead.