What are the economics of bike sharing companies?
Interest is their profitability mechanism. Pretty much everything else is a cost.

Let's make an example:


I start a bike share business.
I buy 100 bikes. Costs me \$10K.
I make the fee of using the bikes so cheap, lots and lots of people have no problem using them as its so cheap.


Here is the catch: To cover my liability (lost bikes, stolen, wrecked, etc.) there is a  one time fee (before you can use any of my bikes) that is a 100% REFUNDABLE security deposit of \$50 (you get your \$50 back whenever you want, so it's still cheap for you).

Everyone's \$50 adds up to quite a lot of money just sitting earning no money. But I want to make money. So what do I do? I invest it in super safe short/medium term bonds or whatever I want at a low/decent interest rate and collect my profit.