The general rule of thumb is to show the text cursor when a section of the page is expected to be copied. The text cursor provides visual feedback that the user can highlight text or input text. So it should be displayed when the design calls for the user to interact with text.
In your example of the tags text, the user really isn't supposed to interact with that text. It's really a label, and while some labels are helpful to be able to copy, here, it just kind of gets in the way of the same cursor being used to type in the tags directly below. It's one of those nit-picky kind of things, but when used properly, can add nice subtleties to the page.
It harkens back to the age of desktop applications. If you see a regular application with an input box and a label above it describing what the input is for, your user is expecting to interact with the input field, not the label, and so the label doesn't have the text cursor, the input does.
If you're looking for a general rule of thumb, I would say use the text cursor whenever the label (or what have you) might be useful for the user to copy or interact with in some way. Otherwise, if the user is not expected to interact with the text in any way except just reading it, I would stick to the default cursor. The user can still copy the text if they need to, so no need to worry about that.