Here's a weird one.

I have a customer with a few domains on the same server. (siteA, siteB, siteC). If I FTP into any domain with another site's username and password, it works.

If I do this in an FTP program
ftp.siteA.com
username_siteB
password_siteB

I login to site B's public_html dir!

Is this normal?

Yep, that's normal. Ftp does not care what URL you are using, it bases its routing off of UserID, not URL.

thanks!