It all averages out, a rush of donations covers the slump when they dry up completely.
That site does say (currently) £10 / £100 per month, so your donation counts towards the total, so suggests the total is £100 per month. You're right, a similar system here wouldn't have to mention any running costs... It's a great suggestion, but for now, with our current size, the costs just aren't enough to really need to be crowdfunded. If that ever changed, the members would be the first to know, If there's ever a concern I'd just drop this place as it's costing money, it'd just never happen. On that site, it's a bit too prominent for my liking. Feels like it's dangling an obligation in members faces.
The issue with closing it when enough money has come in, is that if someone is feeling like donating, but sees it's closed or not needed, then they won't feel their donation will make a difference, then when it is open, it's an implied obligation. It's far better being as completely optional as it is now, to be honest. Call me strange... Many people have.