This would be consistent with makefile -j where, if no argument is given, would select a number of parallel recipes to build itself.
I'm not sure for every make implemn, but quoting the GNU make manpage:
If the -j option is given without an argument, make will not limit the number of jobs that can run simultaneously.
Which looks more like all than auto.
(This typically make make -j a bit violent when compiling cpython.)
I'd say all is not a sane default, and defaulting to auto looks a better choice.
While thinking about it, I bet (it's only a bet, so don't mind me) auto can be faster than all as both run the same quantity of instructions, but in the all case there may be more context switches (or worse, swapping).