1) I have a feeling the probable reason is because they wanted prot warriors to stop using PvE gear in Arenas. In all reality block value doesn't matter THAT much, sure it's marginally better, but if you only have around 13% block then you're only adding like 300 or so damage blocked every 10th hit or so, not that great in the grand scheme of things compared to other stats. It's not bad to have more block value by any means though.
2) Because blizzard doesn't want to give people PERFECT gear, they have never itemized an entire tier level perfectly, and typically won't. Very few pieces are itemized perfectly, but tbh, it's not that big of a deal. Parry itself is alright, the problem is it suffers diminishing returns a lot harder than dodge does, so you have to stack a lot more parry to get as much avoidance as dodge.
TBH, don't worry about avoidance ratings in ICC. Stam and Armor are going to give you your best bang for your buck.