And usually extremely generous. If your deadline has passed, you've twiddled your thumbs a loooong time. I have no problem with expiration policies. They should be held open forever? Not at all unusual for gift cards and such to be charged a monthly fee after one year of non-use until the balance drops to zero - which often gives you another year to waste.
I use istock from time to time. I sell low-end stuff there, so never actually have to buy credits.
To protect people too stupid to use them in a reasonably timely fashion.
I'm fine with consumer protection standards but good grief.