as the site says the glue acts as an additional shock absorber, if they were just bolted around the edges they'd probably rip off on impact
i was looking at these for the SV as i know a guy who has them on his gsx1400 and they look sweet, he had an off and the case covers and frame sliders saves him from a very expensive repair bill, all he has to replace was his handlebars and one mirror after the bike had slid down the road.