It's actually worse than that. They seize your credits if you don't make a call (or text) that uses the credits in six months... it doesn't matter if you've been on skype or not.
It's not really seized, just deactivated, and you can reactivate it any time. I don't know why either, I'm guessing for legal/accounting purposes, like at one point Apple had to charge people for iPod updates: https://arstechnica.com/apple/2009/09/accounting-rules-chang...
Wow, I never knew that was the reason why Apple used to charge for the updates. Thanks for the link.