'We must make a sacrifice...'
Yeah throw that defenseless girl to the monsters, my catapult won't work if we don't do it!
Did I miss something?
They had to retrieve the motorcycle to take it up to the roof. They didn't think they could make it all the way to the police station and figure out some way to get to the roof before they'd be attacked. But they figured that if they created a diversion like that, they could at least get the bike to the roof. They used the bike as part of the catapult.
http://www.rateyourmusic.com/~JrnlofEddieDeezenStudies
I guess it's best not to think too much into it, but I'd say running across the street takes alot less effort than lifting a bike to the roof.
"No man is just a number"