I think it's the galloping incidences of people suing if it all goes tits up. Brembo obviously don't want law suits popping up if they inadvertantly leave any loopholes in their instructions on how to service brakes - and they've got a point. Amateur mechanic's abilities can range from better than a professional to absolute bodge merchant and how would you tell who you had just sold a repair kit too?
I don't think you have to get a whole new calliper though. I have heard that these guys
http://www.gprdirect.com/epages/gprdirect.storefront can service them for you. They're Brembo distributors and I'm sure that people on here have had them rebuild and service their brakes. I've not used them for servicing but I did buy a set of billet callipers from them and they're top blokes.
Jools
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