This is standard practice for any company, and anyone who's ever registered a domain will know that domain sellers always encourage users to do stuff like this too. It works very well at keeping traffic on your site and away from phishers who abuse misspelling domains to trick people with short attention spans into giving their usernames and passwords.
Is it standard practice for a company to acquire misspelled domain names that correspond to a product that's been around for more than ten years and an URL that was originally registered more than thirteen years ago?
While I understand that purchasing misspellings is a good practice to begin with, I find it interesting that Apple chose to make this move now. After all, they've been able to afford to purchase them for years, and, especially with iTunes Match and iCloud coming, perhaps they're developing iTunes in the cloud and they anticipate a that a lot more fingers will be trying to peck out itunes.com.