Seems they still have their old site running as well !! Try this one for them ( I know it refers to IMC but just happened to be how its saved on my 'Favs').
http://www.emailcasts.com/rainbow_ai...IMC_Rating.htm
Jo-ey, do you hold any instrument rating? I think you will find that it may be something to do with that as I do not have a restiction when I hire. Alternatively you could offer to do dual time with their instructors to show to them you arent going to stack their aircraft. Even before I held an IR there was never any mention of 600nm so may be a new requirement. I would drop them an email and find out exactlt what its for and how to lift the restriction.
Julian.