The decision was deliberate and matches what we've done elsewhere in Python.   The nearest kin to Counters are sets/frozensets which also behave the same way:

    >>> class S(set):
            pass
    >>> type(S('abc') | S('cdef'))
    <class 'set'>

Mark Dickinson articulated the reasons clearly.   Python has been consistent about this decision from the outset (see array.array or fractions.Fraction for example).

Marking this as "not a bug" and closing.