Exactly. If this were implemented, it would affect me one of two ways*

1) The cost variation would be small enough that I'd ignore it.

2) The cost would sometimes be high enough to dissuade me from taking a bike where I wanted to go, in which case I would quit the program completely. I don't want to have to change my travel plans at the last minute. Even if the extra cost were non-trivial but acceptable, the mental overhead of having to decide whether it's worth it would make the program much less appealing.

* If I were actually participating in a bike share program. Currently I own my own bike, but I could see using a bike share program in the future.