Why does the “Back” (or “Return”) arrow become grayed out and inoperable

Why does the “Back” (or “Return”) arrow become grayed out and inoperable when I follow a link to a new page? Using the History menu to return to the previous page involves an extra step. (I don’t want to turn on “gestures” on my trackpad as a possible alternative, and I don’t normally use tabs.) I can see that some merchants might not want you to leave their Web sites once you get there, but it happens at more reputable sites too. What’s happening?