This is old news. It happened to me a few times in the past, as well. If you search for a good domain on GoDaddy and find it is available and you do not register it at that moment, the chances are good that GoDaddy will be auctioning it the next day. For a while they were ending up at Melbourne IT, a registrar in Australia, which made me think that they had some kind of deal with GoDaddy.
GoDaddy was caught doing this a few years ago about the same time that Network Solutions was caught doing the same. I remember that Networks Solutions claimed that they were holding the domains to protect them for their customers. Ri-i-i-i-i-ght!
I no longer check for the availability of a domain at a registrar. I have never had a problem using the Domain Tools Bulk Check.
Bulk Check | Bulk Domain Availability Checker | DomainTools
There is nothing illegal or unethical about a registrar grabbing good domains. But the way that they do it is suspicious and does not build trust.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin