You're right. The deselection doesnt happen if I click the titlebar, but it does if I click the content area (that doesnt happen in OS X, I havent checked any Linux window managers).
Of course it does, when you click the content area you reposition the cursor and thus deselects the text. The alternative would be to not be able to reposition the cursor when you bring focus to the window which presents other "quirks" if you look at it from other angles (and it of course needs to be consistent, if you can't reposition the cursor why would you be able to press a button if the window isn't active? etc.).
OS X is quite unique in this and probably stems from the use of common menus for different windows, which also makes it uniquely inadequate for handling multiple monitors. I just can't take a WM that doesn't handle multiple monitors well seriously.
I, and a host of others, work fine with multiple monitors..
I'm sure if people were presented with both options; regain focus and lose selection vs keepi g selection, they'd choose the keep selection. It's as if someone used the computer and said "you know what would be useful?..."