The committee has some discretion to enforce or not enforce. A committee might be of the view that the frequency or quantity of the washing in this case is so minor as to not be worth enforcing. Or they might have some other reason, which may or may not be reasonable. Perhaps they are just prioritising the limited time and energy of a group of volunteers to something they consider more important than the offence caused to some by visible smalls?
The committee of which I was a member till recently had one member opposed to enforcing a particular instance of a breach of a rule (ACT-speak for by-law) but the rest of the committee supported enforcement and proceeded. The minutes recorded the one dissenting view. So, for a start, the committee members could insist that minutes record who supported and who didn’t.
The committee only has discretion in the absence of a general meeting resolution directing it. So, if you are sufficiently bothered by washing (I am generally unconcerned by washing on balconies), you could put a motion to a general meeting that the OC directs the committee to enforce this by-law in all circumstances. That would test whether a majority of owners feel this is necessary or reasonable.