That might have been the preference of the venue or the speaker. Sorry to say it, but some hearing people are distracted by the interpreter and will be dicks about it.
I work at a hospital where we often will use interpreters. My personal preference as the hearing person is for the interpreter to be slightly behind me off to one side. I really don't like it when the interpreter is right next to the Deaf person for exactly the reason that you're describing. The conversation is between me and the consumer. It gets annoying when everyone is breaking eye contact to switch back and forth between us.
