In Swedish and, I'm led to believe, British supermarkets, parsnips are often sold pre-clipped. Topped and tailed.  If I have a choice, I prefer to buy my veg with a few leaves on so I can see how fresh it is. So I have rather cynically assumed that this is done to hide the age of the product. Whether or not this is the case, I wonder why the same trimming service is not extended to carrots.  Anyone able to shed some light?  Update: Quite often just the tip of the parsnip is trimmed. This appears to be a global phenomenon; here are some parsnips from an Australian market.    The two answers I've so far received are good, but don't explain why the tip might also be trimmed. 
The answer to why parsnips are topped is answered satisfactorily by both Steven and Jefromi.  So why are parsnips also trimmed at the tail? This is obvious to anyone who is familiar with the unadulterated specimen.   Behold the untrimmed parsnip!    The full root is of quite variable length and sometimes long enough to be unwieldy in packaging. If you're not happy about your parsnips being tailed, you can do as I once saw Jamie Oliver do and carve pointed tips back onto your parsnips for presentation purposes.  To summarise: There appears to be no chicanery here. The tops are trimmed to increase shelf life. While sellers, most likely, do this for their own benefit, you might just as well top parsnips yourself if you are not going to use them directly. This goes for both parsnips and carrots.  Parsnips are tailed, I hold, simply to make them a more manageable product. For carrots this is unnecessary; the carrot varieties I have seen either appear to lack this long thin tail entirely, or have a very thin hairlike tail. 