Is this "junk framing", or appropriate respect for the integrity of
the older building?  Wouldn't it be more correct to at least strip off
the shingles and tie the new roof to the old roof?  Doesn't this
approach risk runoff water getting under the new roof structure?

Couldn't get to the pic, but rather than "appropriate respect," it sounds more likely that the owner was too cheap to have the old shingles ripped off, the contractor was too lazy, or both.  There's a good chance the contractor told the owner he'd save him 50 cents by leaving the old roof in place rather than paying laborers to remove it, and paying tipping fees to get rid of it at a legit (or illegit) dump, and the owner went for it.

 

As for runoff, as long as the new roof is flashed properly (and that may be a big if), there shouldn't be a problem leaving the old ashfault shingle in place.  It's relatively uniform and compact dimension-wise, unlike old wood shingle, so it should be OK,  But I'd agree it'd be better to rip it out. They may be planning to lay down an entire new roof on new deck at the addition, and over the old ashfault on the old part of the building.

 

Ralph