Lets look at the valuations first, in any open market where prices are flexible buyers tend to feel folks ask to much and sellers tend to thing folks offer too little. If you walk away from a deal feeling like you didn't pay enough for something or that you sold it for too much money, then that market is out of balance. There are three sharks, I've not seen any credible evidence that they have any anticompetitive agreements in place, basically they seem able (and willing) to jump into another shark's negotiation and screw it up to gain leverage. So of the three of them the valuations are no doubt precisely correct. You might find an investor who is willing to give a higher valuation, but if you did that with full disclosure that these three would not agree to a higher valuation you might find it hard to justify. This disparity often arises from emotional involvement with the concept or the company. The wine-by-the-glass[1][2] guy is a classic example. They disagreed on value, he turned them down, and kept cranking. Were they 'wrong' ? No, the valuation to them was $x and to the owner $y, but they see a different business than the owner does.
As for arrogance, these folks seem to get a lot of pitches, I'm sure being on TV doesn't help with reducing the volume. The weird thing is, they still listen. You can hear that in their analysis. Arrogance would have them not listening and just telling the person pitching is wrong, but they give them a chance to explain, sometimes repeatedly. Its clear that some of the people coming on the show have done exactly zero homework, haven't even done a network search for starting a business or what it takes to run a business. I am really impatient with folks like that but the sharks take it in stride.
[1] http://www.copadivino.com/
[2] http://www.aoltv.com/2011/03/21/shark-tank-inventor-wine-by-... WARNING HUFFPO Category 5 MONETIZATION
I don't think their valuations are crazy, they're aggressive but they usually have plausible justifications for them. What is crazy is the gulf between what they do and the kind of startups here - just imagine a couple tweeners with half an iphone app successfully raising a million bucks on Shark Tank.