If you check out a bike, and it disappears, you're responsible for it.

So instead of a $100 headache, you now have a $1500 headache.

You do avoid maintenance costs, of course. Which are $0/year for many bicyclists.

These things are self locking, so you release it as soon as you leave it unattended and are not liable anymore. If you had to look for a docking station, it would be not practical.

How does zipcar handle car theft?

Zipcar buys insurance and GPS/cellular tracking devices.

Nobody who bikes regularly or serious owns a $100 bike. Good bikes start at around $1500, which is why bikes are such frequent targets for theft.

Sounds like a No True Scotsman fallacy to me. I've commuted daily for years, plus occasional longer trips, on a used bike that cost $150 (plus another $100 to add lock, rear rack, fenders and lights). Maintenance may be a little bit more with a used bike, but it rides great.

My $400 bike does me well enough.