Wow. That's new. We raced 2 or 3 years ago at VoS, using TT bikes. This looks like a promoter limitation, not an official rule.
Personally, I think it's idiotic. Like the amount of training they're going to get from a 30-minute race is somehow more beneficial if they're on road bikes. Idiots.
This is coming from USA Cycling. The US Cycling coaches that select who go to Europe, be on the National team and the invited positions at Worlds have decided this is how they want to see riders compete for selection.
I do like not having to drag two bikes. But we spent a lot of time practicing a TT - in a TT position, and it is on TT bikes in TT position at Nationals.