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I think the rationale behind this isn't entirely malicious. Consider the fact that domains are valuable property. If you were in the process of buying a domain, and had to take the time to fill our user information, credit card information and all of the textboxes that they make you fill out (particularly if you are a first time user), there is a good chance that someone from another registrar could snatch the domain out from under you simply because they were a faster typer, or had previously registered. In this way, if you do a WHOIS search with NSI, the name is locked for a short time to allow you to complete your transaction, and to disallow anyone else who may be following in your tracks to buy you your name before you can.
It makes sense, and I'm frankly surprised that no one hasn't noticed this before. I for one am glad that someone peeking over my shoulder can buy a domain from their iPhone before I can finish clicking "buy".