Is it to discourage people from being obnoxious?
This has been discussed several times over on MetaSO - this is a good example.
It's worth contrasting the design of comment @-notifications with those of chat, where you can notify multiple users in a single message:
- Comments always notify the post-author, regardless of @... Chat messages appear to anyone in the room.
- Comments can @-notify anyone who has ever commented / edited the post... Chat messages can notify anyone who has been in the room recently.
- Comment notifications persist in your global SE inbox... Chat notifications produce a small indicator within chat, with an entry in the SE inbox only if you don't respond.
- You can only leave one comment every 15 seconds... You can post multiple chat messages within that timespan.
In short, chat messages will only notify people who are probably expecting them, having recently joined a chat room for the purpose of chatting. Comments might notify people who've long ago forgotten about the topic. Making it easier to mass-notify multiple users via comments has a much larger "harassment" potential.