That's observant of you, but you've conveniently left out:
:
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Some readers might even consider sentences 2 and 3 to break a rule.
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It's all down to interpretation. There's no authoritarian body that denotes when the use of commas is right or wrong. If you and OANST have a problem with it, grow enough prominence amongst literary circles to declare yourselves head of some sort of Grammar Law Body and publish your new rules. It can be a fun little summer project for you guys.