There's no catch - other than the games defaulting to showing you other titles from Digital Chocolate on each game's main menu. Note also that the games are Java-based - if your device doesn't show the game full-screen then you may wish to go into Settings | App Manager | Installed apps | {app name} | Suite settings to turn off the intrusive 'Game and navigation keys'.
Digital Chocolate relationship with Nokia stretches back to the second generation of the N-Gage, with their "Cafe" titles proving to be strong implementations of classic games; with the cancellation of N-Gage they moved to the Ovi Store and the full catalogue of games became available in the store, and met with a positive reception (here's Ovi Gaming's review of Dictator Defense).
And there's nothing wrong with a promotion - Digital Chocolate obviously get a lot of downloads out of it, and the name recognition for the company is going to be worth any potential loss of income. If you have a percentage of these new users buying a Digital Chocolate title in the future then in all likleyhood they'll still have a made a profit. Customer Acquisition is as valid a business expense as development.
Update: The promotion is not running in every country where the Ovi Store has a presence, so please do double-check the price before you hit the download button.