Can an employer make a salaried employee work 7 days a week?

Yep - assuming that the job requires it. In the engineering profession it is not uncommon for staff to have to have to work every day for several weeks in a row to meet project deadlines. Same with personnel at law firms, when a trial or major filing is coming up. In retail businesses store managers may be required to be on site for extended days during the holiday crush. I'm sure there are plenty of other examples.

Heck, I'm an hourly employee, and I am currently on a 7 day stretch. If I want to keep my job, in this tough economy, I do what the schedule tells me to do.