I asked this question at Bol.com, one of the biggest webshops in the Netherlands. Their response was to prevent being flooded with support emails about forgotten passwords. They then curiously ignored my inquiry about the password reset feature which just emails you a reset link when you have forgotten your password.
I've concluded that it's most likely a decision from the management team. Alternatively it might be that they don't hash passwords and use this to prevent their database from being flooded (even though you can have a longer username, so this seems unlikely). They also did not respond to a question about whether they hash passwords (you'd think if everything is alright there would be no reason not to reply).