Giving pre-orders special skins / hats / etc would be fine with me, but even giving them unlocks that non-preorders would have to level up for would be unbalanced and unfair, as is giving them items that items that nobody else can even get. (That said, if anyone who preorders gets the same thing, e.g. it's all in the Limited Edition, I wouldn't complain too much - but that doesn't seem to be what they're doing now)
I've already pre-ordered the Limited Edition, and from how this appears to be working in the UK, the Physical Warfare Pack costs extra above and beyond what the Limited Edition costs, so you have to cancel your existing pre-order and place a new one for a different, more expensive, product in order to get those added items.
I note that right now (and for the past several months) the Limited Edition's description on Amazon says:
Limited Edition Features
Pre-order Battlefield 3 - Limited Edition to get Back to Karkand expansion pack at no extra charge. The Limited Edition includes:
Four legendary maps from Battlefield 2, including Strike at Karkand, Wake Island, Gulf of Oman, and Sharqui Peninsula. Combined, these maps have accumulated over 25,000 years of play-time.
Ten iconic Battlefield 2 weapons reworked with the Frostbite 2 Engine.
Four brand new vehicles.
Unique rewards, Achievements/Trophies, and more.
How much do you want to bet that the cake is a lie?