Comment 2 for bug 1528090

What Robert described is correct. This is reflected in the language in the manual: "If you have not allowed comments in general, you can still choose to allow them for a certain person or group of people who are in your access list."

In other words, the individual share row "Allow" and "Moderate" settings are only displayed if you have disabled "Allow Comments" on the page level. Because if you've enabled it on the page level, then it already applies to everybody. I just tested it and it still works this way in Mahara 15.10.

That said, there's certainly room for a UI redesign here (maybe for 16.10). New people I show this functionality to are always a bit flummoxed by those appearing/disappearing checkboxes.