George Bernard Shaw said, "He who can, does. He who cannot, teaches." Are commas properly used in these sentences? In my mind, the comma in each sentence improperly separates the noun ("he") from the verbs ("does" and "teaches"). The other phrases ("who can" and "who cannot") are dependent clauses that could theoretically be removed.
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If you think that "who can" and "who cannot" can each be removed then you should use two parentethical commas as in "He, who can, does. He, who cannot, teaches." This could suggest you might already know who these two men are.
But I doubt Shaw meant this. So the prescriptive alternative is have no commas as in "He who can does. He who cannot teaches." The problem with this is that it is prone to being misread initially and causing a stumble: it might be even worse if it was in the plural as "They who can do. They who cannot teach."
So the version you quote may break prescriptive rules, but it provides an aid to reading by indicating that can and especially cannot do not apply to the following verbs. In my view that makes the use of such single commas helpful rather than wrong.
As a reader, I treat commas like breath marks in a line of music -- as a directive to break the phrase, and breathe.
To continue the analogy, the composer, Shaw, clearly intended that the phrases be broken where he placed the commas:
As a musician, I try to divine the composer's intent from whatever clues he or she bothered to leave. (Some bother more than others.)
As a reader trying to give life to the author's words (especially if I'm reading out loud), I do the same with the somewhat less rich notation available in the written word.
The commas are properly used according to Shaw's intent.
One way of looking at the problem is to punctuate the relative clauses who can and who cannot in accordance with the convention of using commas to set off supplementary relative clauses but not integrated ones. If you regard the relative clauses in these two sentences as supplementary, then you can write He, who can, does. He, who cannot, teaches. If you regard thems as integrated, you can write He who can does. He who cannot teaches. However . . .
There’s another way of looking at it. Larry Trask identifies what he calls the ‘gapping comma to show that one or more words have been left out when the missing words would simply repeat the words already used earlier in the same sentence.’ He gives as an example:
The problem with the OP’s examples is that the gap in the first sentence precedes rather than follows the word that might otherwise appear. In full it would be He who can do does. The second of the two sentences is a little different, because the missing word, do, is assumed. In full it would be He who cannot do teaches. So, what to do? I’d use the gapping comma in each sentence, even though Trask doesn’t cover this particular case. As he writes at the end of the section:
Shaw was notoriously unconventional in punctuation (and would have been in spelling, if his editors/readers had allowed it). He used commas, semicolons and colons simply as pauses, as in the speech from Arms and the Man which starts
It looks indefensible, but when read aloud the pauses produce exactly the effect Shaw intended, which is after all the important thing in a play. Similarly, the strict meaning of your quotation could be put into "Who can does; who cannot teaches." But the pronouns and the commas indicate the interpretation,and make the sentiment comprehensible.