There are perfectly valid other reasons: your employees normally interact with the same group of people from their local area, minimizing the likelihood that they'll be exposed to novel contagions and making it easier to slow the spread of anything they do contract. Mandating that they attend a conference that takes thousands of people from all over the world, puts them on airplanes with notoriously poor air circulation with a bunch of random strangers, and then puts all of these travelers together in a crowded conference space is not like sending them down the block to pick up office supplies.
Yes, the risk is still relatively low. It's still not zero, but it's also largely avoidable since this kind of show is optional in a world of pervasive electronic communication. Conferences are notorious for spreading infectious disease, so this is one of the things we can do to minimize personal risk and slow the spread of something new. It's neither rocket science nor a conspiracy theory.