Some cruiselines may have implemented it as a convenience for the passenger, and a preference on their part. But home computers are not yet so pervasive that a law would require such registration. It might even be considered discrmination against folks who for reasons of economy, or even religious convictions, don't use computers. Can you say class action? Many attorneys could.
One of my hobbies is communication history. The closest historic precedent i can think of is touchtone dialing. DTMF controlled tel switching has been available in many areas starting about 45 years ago. In fact, it's only 25 years since the last hand cranked system went off line. So for a few years, within a radius of 200 miles from my home, there were 3 different dialing technologies. As of 10 years ago, there was a community in my state where DTMF was not yet available. By then there were also many businesses, perhaps a majority, using Touchtone menus to pre-screen calls. Those businesses had to reftrofit their menus to include a zero option to reach a live operator to accomodate customers in regions where pre-DTMF technologies were still in use.
PCs in the home are still that new, despite the hype. I'm not even sure cruiselines could fast track the boarding of passengers, with online pre-registration as a sole criterion. It will be interesting to see what develops.