If you are informed ahead of time when the raid is being formed that certain items are reserved by those who took the time to organise the raid, I personally don't see a problem with that. As many players here are pointing out, you are always free to choose not to attend the raid, or to lead your own raid in hopes of getting the item you want.

Speaking as someone who has both lead and attended many "open" raids, there are a multitude of loot rules that I have seen in place that also seem fair. For example, the "last piece" rule where players with 7/8 pieces of an item set would get priority on their last piece, etc.

Anyhow, that's what I think ;) You are of course free to disagree!