Would you mind a little tough love? You can ponder who reported you, but what you did by draining your water that way, if doing so does in fact violate local ordinances, was not cool and you can't really fault a neighbor for calling you out on it.
Depending on where the street run off goes, yes, you could have definitely impacted your local environment. Your city is most likely concerned with the salt or the chlorine. Chlorine could have, probably should have, been neutralized before you dumped it. That might have been what they were after. If the run off goes to a river or lake or planted area, salt could kill off all kinds of things. Cities can mandate that you truck the water out. Sewer systems are designed to handle certain types of waste. Gutters are usually designed to handle rain water, and not much else. They can connect directly and without filtering, to rivers, lakes, ocean, etc.
Sorry, don't mean to beat you up, but you really should have called the city first, and if you're being honest with yourself, you know that, or else you wouldn't have worried about what time of the day you drained your pool, yes?
Don't be mad at your neighbor(s), they have a right to protect their neighborhood, just as you do. As you should do. You'll either get a warning or a fine. Take your lumps. A lesson for next time...
No-fun-allowin'-Dirk