AbdulAlhazred
Legend
Exactly my point - they don't both apply. But, apparently I wasn't clear about 'they.' In this case, I mean to say that you can't simply drop an equipped shield, just like you can't simply pick up a shield and call it equipped. Allowing dropping a shield that is equipped as a free action is like allowing dropping something out of your backpack as a free action (without the minor to retrieve it first).
Well, the thing is its simply not necessary for the game to impose a standard action as the cost of dropping your shield. In fact they say it costs a standard action to "stow your shield" which is actually doing the PC a favor since otherwise it would cost 2 minor actions (drop and put in backpack) which is a higher cost. If you just want to dump the shield though, its a minor action.
Think of it this way. The straps you put your arm through on a shield are not tight. They are just loops of material and then you grip the handle. Let go of the handle and the shield will slide right off easily (minor action). Putting it on is harder, you have to get things lined up so your hand goes through the loops, grab the handle and bring the shield into position (standard action). In game mechanics terms that standard action is what stops players from gaming their shields.