It isn't just as simple as removing a competitor for the Apple Watch from the market. Actually for the next couple of years Apple needs to encourage competitors devices into the market. No, Fitbit's crime is that it has failed to jump on the Healthkit bandwagon and as a result its devices threaten Apple's long term domination of the health information market.
And that is what Apple's Watch and Healthkit are all about.
Here's the scenario. Fitness band manufacturers are being encouraged (some would say blackmailed, given the Fitbit case) to integrate with Healthkit. In the short term this probably looks very attractive to those manufacturers, they can do away with much of the responsibility for managing and storing health information and hand it over to Apple.
This is a dangerous place to be for them. Having handed ownership of that information over to Apple they have no lever to keep customers coming back to their devices. Next time around that customer will buy something else. And in a couple of years there will be Apple Watches at all price points that matter, meaning that most customers will be buying from Apple. Those fitness band companies are going to be badly hurt when that happens.
However I don't think that pushing sales of the Apple Watch is the prime mover behind this process. After all, Apple are likely to be able to sell every watch it makes. The more important factor here for Apple is lock-in. Once you've stored your health data in health kit it will be very easy to change your fitness band hardware, but leaving Apple becomes very difficult indeed. And above all else Apple prizes locked in customers.
By refusing to integrate with Healthkit, Fitbit has put itself in a position where it is a threat to Apple's strategy. As a result Apple is going to make sure that Fitbit dies. No Apple Store presence (physical or online), limited promotion in the App Store, and strong reinforcement of the message that Fitbit is no longer Apple's friend will all go a long way to marginalising the company.
As a result I'd expect to find that Fitbit is working flatout on getting itself integrated with Healthkit as soon as humanely possible.
Integrate or die.