Yeah I've heard of HP and others doing this. Although there could be any number of reasons for lower scores after overclocking, its my understanding that HP cripples the BIOS to disallow overclocking. My guess is this is a liability issue, whereas they had computers being returned after failing post-overclock so they decided to just lock the BIOS and disallow OC.
If interested in overclocking, I suggest you get a motherboard that is more overclocking friendly. I'd look at a 990X board, unless multi GPU appeals to you then you might consider a 990FX mobo.