You would typically do point-to-point trips between stations. Bixi Toronto has something like $500-$1000 liability if stolen, as I recall (less if recovered later presumably... the bikes stand out so it'd be rather difficult to pawn).
There are various ways to deal with this, and it can vary between programs. In general, the rider is responsible for ensuring the bicycle is kept in good order.
viaCycle's system allows riders to secure their bicycle between stations, so you're not forced to ride from to specific points until you're ready to check it back into the system. Enforcing a penalty for bikes that get stolen helps reinforce proper locking of the bicycle.
Or do you use these bikes going between destinations which have these bike stations so you don't expose yourself to any risk of leaving unattended?