I am a per diem transcriptionist in a hospital, paid by the line. I have confimed twice with my supervisor, just this week by email, that when I work a holiday I am to be payed for 1.5 lines. My paychecks do not reflect this; just one lump sum for total lines payed that is at my normal rate.

Can they do this?

Have you contacted payroll with the emails in hand to make sure it wasn't just a mistake? Chances are good that they can do this as holidays are not required by law. If it is in a contract of some sort, you may have a contract violation, but if it was just your boss saying they would pay you more, that liekly wouldn't cut it.

We were reminded of this policy by the supervisor at a staff meeting. I then confirmed it by email with her, but my dept is frequently mixed up on what they can and cannot do. Does being per diem add anything different to the equation?

It doesn't really change anything other than many companies exempt per diem employees from benefits such as holiday pay. In that case, the policy wouldn't apply to you. It is entirely company specific though.