The text box is on the master page "Master A". If you Control+Shift click within any page's margins you'll override the frame and it'll come onto that page. The normal use for master text frames is for importing text. You would just Import your text, then click on the page with the placement cursor. Hold nothing down and you'll dump the text onto the current page, and any overflow will be hidden. Hold Option and you can clik, clik, clik.. to your heart's content on individual pages or columns. Hold Shift, and InDesign will dump the text, adding pages until it runs out of text.
You're coming from Quark, right? In QuarkXPress, master items appear on each page as fully editible. In PageMaker, master items were like letterhead -- you can't edit or move them, you can only hide all of them. InDesign's master pages are, IMO, the best of both worlds, with fried onions.