This is a pragmatic choice.  Try searching the tracker for 'cookie comma', and read about the lack of adherence to cookie RFCs by the major browsers.  Specifically, I think issue 1210326 is relevant here, and am closing this as a duplicate of that issue.  If you disagree, I think we'll need examples from real-world browser/server situations where this is an incorrect choice in order to consider changing it.

You will note that the comment block before that equate mentions that it does not follow the RFCs for pragmatic reasons.