Why are parsnips sold pre-clipped?
In the US, carrots are normally sold topless too.

I do understand the desire to see those fresh green leaves on it, but in fact, they're not helping. Since they're still alive, they draw nutrients and water out of the root, decreasing what's left for you. This means they may actually be worse than ones that had the tops left on them, especially if they've been stored for a while. They'll have less flavor and won't be as crisp. If you do buy them with the tops still on, it's a good idea to at least trim them before storing.