I think one of the key elements of a DSB is the fact that it allows anaerobic (unoxygenated) bacteria to flourish. If the flow came from the bottom instead of across the surface, the bed would be oxigenated defeating one of the key elements of the DSB.
Also, should your plumbing burp, you could release all kinds of nasties into your system from the DSB. Its my understanding that they need to be left alone and a major varience in your flow could turn things upside down.. Think "death sand bed" as some refer to them as.
I am no expert in fuges or DSB's. I am in the process of designing and installing one on my system. I have on the otherhand been doing my homework & reading up so I do it right the first time.
I do think its a very good question though and will be interested to see what others have to say.