Not sure I got that - why is cable different in this regard from DSL? I'm pretty sure there is such thing built into adsl/vdsl routers or modems, at least not in Germany.
DSL is provided over a POTS ("plain old telephone service") line. The demarc for POTS, at least in my neighborhood, is a grey box on the side of your house, well upstream of your DSL modem.
I don't know why exactly it worked out that way, but the telephone system is designed to deal with consumer electronics and the coax cable system is not.
It didn't used to be - until the federal government busted AT&T's monopoly, it was illegal to connect non-AT&T equipment to the AT&T network, at least electrically. This is why in old movies you see people dialing ISPs by hand with a phone and then setting the handset down on a cradle - the cradle was the modem, and it interfaced with the telephone network acoustically. The demarc (at least as we know it today) was presumably introduced after the government told AT&T they had to let people have their own networks, rather than considering everything up through the phones to be part of the telco network.
The demarc for your DSL is the modem. If you provide your own modem you still are subject to AT&T's rules. You cannot legally connect to sensitive national infrastructure (like the phone network) without the provider being responsible for your access.
As far as connecting equipment, that significantly predates divestiture. Most people point to Carterfone as the salient decision that changed this: http://en.wikipedia.org/wiki/Carterfone
In reality, there was actually a prior decision which I can't seem to find right now where AT&T refused to allow people to put plastic privacy guards over the mouth piece on the grounds that it would damage the phone network. Ludicrous, but sensible from an economic standpoint.
