Why is USB Battery Charging v1.2 compatible with v1.1, or is it?
I did miss something. Upon further reading (also of this answer), I found that the spec requires a little more smarts of a DCP than I initially thought.

BC 1.2 Paragraph 4.1:


  A Dedicated Charging Port shall go into current limit mode at some
  current within the range of I DCHG . A Charging Downstream Port is not
  required to go into current limit mode, but may do so at some current
  within the range of I CDP . In current limit mode, the output voltage
  of a Charging Port shall drop to a level that allows the Charging Port
  to continue outputting its maximum current.


The 1.5 / 5 A is the max current that a DCP is allowed to supply, not that it must supply.
Chargers are allowed to be weaker than that, and are actually required to implement current limiting to prevent dangerous situations. The consuming device is expected to either suffer the voltage drop, or detect it and start drawing less.