Yes, blocked sidewalks is the issue. And while I'm glad the author suggests a remedy — of creating designated parking zones — this is essentially describing dockable bikes. In my experience, docked bikes do not solve the convenience and availability problem well enough.
I've used dockless bikes recently in both Europe and Asia, and in the cities of both, dockless bikes are being literally strewn about the place. Flower-beds, narrow sidewalks, pedestrian junctions, private grounds, etc. are all being used as dumping grounds for users' dockless bike destinations. I've even been on wild-goose-chases after multiple consecutive 'available' bikes, only to realise that they must be literally taken inside peoples' property and homes.
Why?
For the former, I'm pretty sure it's simply a 'not my problem / not my bike' issue. Private bike owners usually have to lock their bikes up somewhere out of the way, and generally seem more respectful of their bikes' surroundings. With dockless bikes, it's just to easy to dump your bike in the middle of the sidewalk and not have to deal with any consequences. Certain bike hire companies are trying to deal with this by incentivizing certain parking spaces; however, why would you bother getting +10.CoolBikeXP for going out of your way when you can chuck your bike on someone's lawn right outside your office?
And as for the latter, because people seem to be getting away with it. People are so lovely [/s] that they'd seriously consider just keeping a rental bike inside their home, thus ruining the service for nearby users.
I want dockless bikes to be the future, and I think they should be, but we really need to fix users' docking behaviour, or else we'll end up with tragedy-of-the-commons[0] type consequences once enough people start using them.
[0] — this is a tenuous analogy, but for lack of better phrasing, I'm referring to the ultimate widespread abuse of potential public / open docking spaces.