For 15K users protect option is supposed to work as described here:
the question must have at least one answer by a new user aka a user with < 10 rep on the site (which is the required rep limit to answer protected questions)
This answer made me wonder if there is a off-by-one error in calculation somewhere. At the moment of writing this comment its author has only 9 rep on site (6 from one answer and 3 from one question) but protection link isn't displayed to 15K users.
I additionally checked that question is more than a day old: per FAQ this is enough for protect option to become available.