open('foo','ab+')

[snippsat]

if + means read and write, why can't it be used alone?

Because they have written it so + don't work alone.
I guess no need for one mode for + alone,as r w a in combination with + make more sense.
If look at source line 323

case '+':
    if (plus)
        goto bad_mode;

So it goes to bad mode.

>>> open('foo.txt', '+')
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
ValueError: Must have exactly one of create/read/write/append mode and at most one plus