The party benefiting from the transaction should be the one with the burden of converting. Why should the admins do it... hmm... maybe for money that's being offered? I mean think about it, the easier it is to donate, the more likely people are going to do it. If that means you need to set up infrastructure for accepting it (which shouldn't take long and is, to the best of my knowledge, free), it makes sense to do so.
If I ran a site, and someone was offering me money if I accepted their currency, which is easy to convert into my own, I'd probably do it!