I think you misunderstand the rules here...
Flyers have a rule where if they are getting shot at, you shoot at BS1.
Skyfire USR gives you the ability to shoot at flyers and flying monsters at your normal BS, but everything else at BS1 (except skimmers)
Scourges dont have skyfire rule, so they shoot flyers at BS1. Infact the only thing that can have skyfire with our army is our own flyers (all flyers pick whether they have the Skyfire USR or not)
Skimmers are not protected by this rule. So everything shoots them at normal BS, also things with skyfire (so hydras) still shoot skimmers at normal BS for some reason... Personally I dont know why they put that in there, a skimmer should be able to get low enough not to be hit by AA fire, but whatever, its GW...