Spam is repeated or unwanted communication. Some people enjoy getting community coupons on their mailbox and some people prefer to only receive letters directly addressed to them. It’s a preference–neither is right or wrong. The preference is subjective, and in the wow scenario, dictated by the community through the report system.
You seem to be the one failing to understand that any spammy chat is spam, regardless of frequency. You continue to define spammy chat as “frequently posted information,” which would include simply saying “hi” over and over again, for example whereas spam is unwanted communication at any frequency interval.
That means when you post, “rare at X,Y coordinate” and then “rare dead” many times or one time, both can be interpreted as spam. Telling people that they should leave the chat if they’re bothered by it, or telling people that you’re providing them useful information even if they don’t want it, does not change the fact that it’s unwanted communication in the channel you’re posting it.
The fact that a few of you refuse to respect the wishes of the community, evidenced by the fact the community is reporting your communication as spam, is further evidence that what you’re doing is spamming the channel. If someone is following you around offering you 10K gold in game, if you don’t want it then it’s harassment if that person follows you all through the game trying to give it to you regardless of how helpful it would be to you in the abstract.
In practical terms, the addon is useless. Due to sharding and speed of execution of the rares, if you aren’t in the group broadcasting the location (and you’ll need to be right on top of the rare, for that matter) then spamming it into the channel doesn’t help anyone. The realistic targets of the communication are group members and, as such, it should be restricted to group chat.
If none of that manages to penetrate your skull, consider it philosophically: it’s untenable to have a system where everyone is spamming their group’s information into the general channel: all parties, PvP raids, PvE raids, and farm groups. It’d be a festival of nonsense.
These are just some of the reasons why arguing that your add-on is only really putting a few lines of text into the chat channel so it can’t possibly be spam is a ridiculous position.