You've offered a linguistically prescriptive interpretation that ignores nuance and flexibility of language.
While it may be the case that the canonical "point" of parenthesis is what you've described, the purpose only remains for as long as we culturally accept that definition.
The usage of parenthesis as an aside to indicate a certain emotion somewhat playfully is not only acceptable, it's entered into the cultural zeitgeist. We all understand the meaning. The sentence is complete -- regardless of the rules you've cited -- because the reader doesn't eliminate the parenthesis from their context like a robot, and we have a common understanding of what the aside is trying to communicate.
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Nitpick to your nitpick:
You've offered a linguistically prescriptive interpretation that ignores nuance and flexibility of language.
While it may be the case that the canonical "point" of parenthesis is what you've described, the purpose only remains for as long as we culturally accept that definition.
The usage of parenthesis as an aside to indicate a certain emotion somewhat playfully is not only acceptable, it's entered into the cultural zeitgeist. We all understand the meaning. The sentence is complete -- regardless of the rules you've cited -- because the reader doesn't eliminate the parenthesis from their context like a robot, and we have a common understanding of what the aside is trying to communicate.