Have I got this straight?
Workers are scheduled to attend industry conference.
They work hard to make sure they are caught up on their work.
They make sure that what wasn't finished is covered by co-workers.
All deadlines are met.
They got in trouble for this.
I agree with you, this is absurd. I think you have a rat in your office who pulled a "s/he got to go to the conference and I had to do all the work" ain't-I-a-martyr -look-at-me bull. You know those kind of people who only look good when they can make other people look bad.
I'd talk to HR about it. Ask for a clarification on the policy. If it makes you nervous imagine what it might be like for people further down the chain of command.