Many manufacturers in this industry like to "protect" their territories. Tweeter continues, though at a slower pace now, to buy up local chains and add new stores everywhere they go. They, the manufacturers, also put a great deal of emphasis on salesperson knowledge of the product. They're likely afraid, and rightfully so, about the high turnover of salespeople in a big chain like Tweeter, though they do take quite a bit of time to train their sales staff.
These manufacturers simply like to support the smaller "boutique" type of dealer as they feel it better represents their products.
As for Tweeter dropping Sonus Faber, it wouldn't surprise me after they dropped Vienna Acoustics. I'd bet it was an all-for-one deal for Sumiko and Tweeter's long-term direction doesn't provide for the type of product that Tweeter currently carries. Tweeter's long-term plans are to go balls-out towards whole-house custom installation and away from selling black boxes. Vienna Acoustics' Webern line would seem to fit nicely but perhaps they feel that even that is too far out-of-reach for their target customers. For years Tweeter (and other big-box mid-fi chains) have experienced severly declining sales #'s from the audio side of the business which is largely made up of speakers. The wow factor is now on DVD players and plasma TV's and the sales #'s reflect that.
Their free-standing speaker selection will continue to dwindle as well as their selection of "old-school" rear projection and direct view TVs. In-wall/ceiling speakers and plasma/LCD/DLP TVs are where they feel the market's headed along with everything else that comes along with it, media servers, lighting controls, and other home automation type products.
I don't think we'll see them become another Best Buy or Circuit City in the quality of gear department, but they're poised to become the largest retailer of whole-home custom-install specialty A/V the country has ever seen.
It should be interesting to see how things unfold.