Wizards published this news blurb on their site this morning:

A few things to note:

Grand Prixes are run by regional tournament organizers, and so it isn't Wizards of the Coast running the tournament, however they are the ones providing the prize. It is most likely the tournament organizers who were contacted by local officials concerning the prize payout.

This is also a common issue for the smaller tournaments in regions. Stores can be concerned running cash tournaments as, in most regions, Magic the Gathering is considered a game of chance rather than skill. Stores will usually run tournaments with cash prizes as they are too small for the authorities to concern themselves over. Others will have gotten the proper authorization and paperwork together to avoid any issue with the law.

This also is tied to the situation with Magic Online as some regions are unable to exchange their complete sets, as Wizards will not mail them to their address as doing so may be considered "gambling" due to the inexact letter of the governing laws.