As Josh pointed out, this is how counters are documented and tested to work, so it is an unchangeable decision (i.e. it would break code that relying on the saturating arithmetic feature).  Accordingly, I'm marking this as closed, not a bug.

On the plus side (no pun intended), a counter is just a kind of dictionary so you can easily implement your own behaviors with just a simple for-loop:

   for x in a:
       b[a] += a