In Markdown, readability is emphasised above all else. If CommonMark is to be true to the philosophy of Markdown, readability should take priority over the principle of uniformity.
The use of whitespace can make a document more readable. Creating space around a list is arguably cleaner and makes that list stand out from the surrounding paragraphs (as do spaces between list and header markers, as discussed in depth in this topic).
I would be in favour of making the syntax a little harder (stricter) to write all around if that results in documents that are a little easier to read. By requiring the blank line, you also reduce the amount of variations within a document, making the whole feel more uniform.