Cyclocross Magazine has confirmed with USA Cycling that beyond the well-known rules of no bar ends, no motors, no fixed gears, no protective shields and needing one brake per wheel, Decker raced a completely legal bike and did nothing wrong by USAC rules:

USAC 2013 Rule Book General Rules
1I.Bicycles (page 28)

(e) Bicycles must meet current UCI technical regulations at events that select 17-18, U23 and Elite riders for international competition or national teams. All bicycles used in National Championships (for age 17 and older riders) and NRC races must comply with the current UCI regulations. At the discretion of USA Cycling, UCI rules may be adopted or modified for Masters National Championships.

 

Perhaps the bigger question is whether Decker’s choice was in the spirit of cyclocross, a sport that historically has embraced all types of bikes that were kluged together in a garage, and Cyclocross Magazine’s Facebook followers had diverse and polarizing reactions to Decker’s choice of equipment.

When asked about his bike choice, Decker told Cyclocross Magazine, “It’s in the spirit of racing cyclocross, or ’cross in July at least.” Decker has been racing a lot of Enduro races, and raced the type of bike he’s gotten used to lately. “I’m a big fan of bringing the right tool for a job within the rules. And if the rules say I can bring a mountain bike, I’m bringing the mountain bike.”

Decker didn’t deny his advantage, estimating that he only gained a second per staircase, but given that there were 10 laps, 9 laps on the mountain bike meant 18 seconds in hand. “Running tires me out, run-ups kill me,” Decker confessed. “Running with a bike in my hand, I’m awful, so riding both of those each lap was huge.”

Despite the advantage, Decker was gracious in recalling Page’s performance. “I couldn’t believe how fast Jonathan was going on [the downhills on] a ’cross bike.”

Page agreed on the advantage. “If you can ride stairs, you don’t have to get off, it’s a huge advantage,” he reflected. “When you can bomb the descent, that’s a huge advantage. I don’t know if that’s fair, but…it was a great day in Utah.”