Wait, is it common in the US for brick-and-mortar businesses to charge you extra if you pay by card? That doesn't happen in the UK.
On a €600 laptop or desktop we'd earn maybe €50 to €75. The €12 is a comparatively large fraction of that. We didn't actually make that much more money from more expensive laptops, so for a €1,000 model we'd typically also earn €50 to €75, and with €20 of that going to Visa... yeah. After factoring in costs for sales, warranty, rant of the store, etc. there is not much left.
On one hand you can say "well, it's only €20", but on the other hand if everyone started paying paying by CC we'd pretty much go bankrupt.