Breaking Image Caches

While I was tuning an Image Cache in BlogBridge, fetching different feeds, I noticed one interesting thing. Some feeds, for a reason, impenetrable to me, put random number in a query part of an image source URL, making it different every fetch. To make it more sensible, look at this:

<img xsrc=""/>

I find it difficult to understand why one would use this trick other than to disallow caching of images. Every time an RSS reader application fetches such feed it sees the differences in text and updates a local copy of an article. Then it tries to lookup an image mentioned in the article, which is already in cache, but, thanks to new random number in the URL, the image can’t be found and is downloaded another time… and another. Frankly, BlogBridge is clever enough not to do this, but the whole idea is arrogant, in my opinion.

Why is it done this way? Any ideas?