It is not the "law" that matters here but the "bylaws" of your HOA. I spent many years as a realtor in Florida, the land of HOAs (I think we have more HOAs in South Floirda than the whole country combined). Items that are completely legal were banned in some communities. I've seen HOAs ban skateboards, rollerblades, electric razor scooters, basketball hoops, and even flying a kite. When you purchase a home in the community, you are given a list of bylaws and documents to sign. While it is very likely that "drone" is not actually worded in the bylaws (since drone is a new concept) it definitely talks about nuisance and privacy.
If 3 different HOAs are in your area and the attorney that wrote you the letter is not from your HOA, it is irrelevant. If you are in HOA 1 and someone from HOA 3 complains about you, the attorney for that HOA is going to get the call since he is on retainer with that HOA. HOA 1 isn't going to spend the money to have their HOA write you a letter when the complaint was from a neighboring HOA. You HOA was sent a copy of the letter and was informed. That is standard practice.
As for scare tactics, there are no scare tactics involved here. The HOA received a complaint, they forwarded it to the attorney, the attorney checked to see if you were breaking the law and also if you were breaking any bylaws. This is most likely because someone asked if they could press charges against you. Instead of writing two letters, he wrote the one that states you technically could be breaking a law if you were flying with a camera attached (remember that he doesn't know what you were flying and only is responding to what he was told) and also that you were breaking community bylaws.
Long story short, you have three options:
1. You can stop flying in the community and fly elsewhere.
2. You can continue to fly in which case you will have gone against the request of the HOA and therefore could be forced to pay a fee for every offense. Should you not collect the fee they will put a lien on your home. Should you ignore that, they could eventually own your home (I don't have the time to explain how that works, just know its true). Also, depending on the bylaws of your HOA, you could in fact be arrested.
3. You can move.
I would go with option 1 but seeing your replies to some of the other posts, I have a slight feeling you may be going with option 2. To each their own but before you get in a pissing match with the HOA, you might want to take a step back and see that you don't have a leg to stand on. You can go to your attorney and he can draft some letters to send back to the HOA but the irony is, he will then be billing you for nothing.