It's a strange one and I can only think of 2 reasons to do it:
1. It's a defensive move to protect themselves against users defecting to the Garmin subscription service or
2. It's an offensive move and Strava are about to launch some hardware (or more likely white-label somebody else's).
As a recent Garmin hardware purchaser, I think both of these are doomed to fail, and as a Strava paid subscriber, I really can't wait for my subscription to end.
I suspect it's (1). Strava has a dominant position now, but it does have long term weaknesses and it isn't too big to fail. I think they are being forward looking but taking risky aggressive actions.
One scenario is that it could get reduced to the role of back end database while external services (Trainer road, veloviewer ...) suck the data out and provide value added services for $$ that could be going to Strava.
Another scenario is that hardware suppliers (Garmin, Fitbit ...) end up providing all the services that the end user wants and Strava is rendered irrelevant. The next generation of cyclists may not think "if it ain't on Strava it didn't happen". And once that happens they risk becoming the MySpace of sports data.
So they have decided to take preemptive action. They had a pop at value added providers, and now they're having a pop at Garmin.
It all depends on the myth "if it ain't on Strava it didn't happen". As long as that holds they have something over the hardware manufacturers. A gizmo that doesn't seamlessly sync to Strava is at a big disadvantage. But if that bubble bursts then Garmin and Wahoo etc will just see them as competitors and stop feeding them.
