Is it common practice to publish/sell online versions of board games?
It's actually quite rare. I've never encountered any other company doing so for free or even reduced cost on their own dime, save for the "instructional" modes. I've heard there are a couple others doing so, but I've not encountered them.

Several 3rd party developers have made money by paid subscriptions to their sites and hosting lots of games under license, and others by advertising supported free online play of various games.

I've often seen companies link to such venues.

Days of Wonder used to only give you 6 months access to their online lobby with a game, but it allowed access to all games. Now, they provide access to just the games purchased, but noting that a few unsupported games are able to be accessed free (Including Queen's Necklace online...).

Also note: Many electronic versions of various games are unlicensed - there are a few dozen free versions of Monopoly that are actually not licensed; the official version isn't free.

Keep in mind: most boardgame companies are not staffed with programmers; DoW is an exception.

Update 3/5/2012: I've since noticed others doing so. Amarillo Design Bureau licensed a programmer to create and maintain SFB-Online. A couple other companies have likewise placed electronic versions of their games up as pay to access; most of the "unfree" access now is advertising supported or is in-game payments.