> what's the reasoning behind this?

Fair, per-kilometer pricing. Quoting from the english version of the page linked above[0]:

  When you travel on credit, a boarding fare will be debited from your card when you
  check in. When you check out, the boarding fare will be refunded and you will be 
  charged for the number of kilometres you travelled. If you do not check out, you
  will pay the full boarding fare.

Not everything that looks like a blatant money grab is one. Hanlon's Razor[1] prevails once again.

[0] https://www.ov-chipkaart.nl/reizen/tarieven/instapenbasistar...

[1] http://en.wikipedia.org/wiki/Hanlon's_razor