I am making cupcakes for someone who is going to be a police officer for the supreme court. They (the s.c. police) have a nice seal with an eagle on it. I thought it would look nice as a medallion on a cupcake and be faster than making tiny fondant handcuffs and other accessories.

Would this be legal? Would a Walmart or cake shop print them for me or give me a hard time and refuse to do it?

I think it might depend on how you ask.

If you go and tell them the nice story, they will probably print up your images without any fuss.

Might be worth contacting someone at the police courts to ask permission, if you can get something in writing saying they allow you to print x number of edible images.

You must get permission to use federal seals for non-official business.

http://www.supremecourt.gov/contact/contactus.aspx

"Logos and seals of the Federal Government, Departments, Bureaus, and Independent Agencies are not in the public domain, and cannot be used for other than official business without specific authorization of the agency involved."
Joint Committee on Printing, United States Congress

In my state the police officers in the supreme court are not federal employees. They work for the state, and in the larger cities they work for the City police department.