Apple needed relatively cheap Firewire and Ethernet adapters to cushion the effect of dropping FW and Ethernet from the Retina MacBook Pros.
If you look
at this article you'll see that they've done something clever - interfacing a PCIe ethernet controller directly to the Thunderbolt socket without using the (expensive) Intel thunderbolt controller chip. It doesn't seem that third parties have caught on to that wheeze yet.
However, there's clearly no way of daisy-chaining another TB peripheral or display adapter from this type of 'cut price' TB adapter. The Ethernet and FW adapters are mainly for the benefit of rMBP users, who have 2xThunderbolt + HDMI so that's not such a big deal.
The only real market for a 'simple Thunderbolt-to-USB3 adapter' is for users of 2011 models with no USB3. That's a limited market to start with - now subtract all the Macbook Pro users who don't want to sacrifice their
only external monitor connection, and all the Mac Mini and 22" iMac owners who don't want to lose their second display output, and you're pretty much left with 27" iMac users and Thunderbolt Display owners. Quite a small market, really, and in electronics small market = high price.
If you don't want to lose your only display output, then you want a full-blown TB device with a proper TB controller, a TB-through socket and the internal gubbins to make the TB-out usable with DisplayPort. Plus a TB cable. By the time you paid for that overhead, throwing in a couple of extra ports to make something like the forthcoming Caldigit dock makes sense.
