This happens because it is a default setting in Logic itself, it has nothing to do with the system it happens to be on. The 32 bit setting is (has to be) the initial setting because of 3rd party plugin software compatibility reasons.
If you would start up Logic in 64 bit mode, Logic cannot validate and use any of the 32 bit plugins that a user may well have already installed.
That would render the 32 bit bridge, which is there to accommodate such 32 bit plugins in 64 bits Logic, useless, as logic cannot "offer" any 32 bit plugins to the bridge. For this very reason (the one time plugin validation by Logic) you will also have to start up Logic 32 bit mode every time you install any 32 bit plugin - and there are enough of 'm about.
But, all this is not relevant if you have no 32 bit AU plugins already installed and never plan to do so. You can set Logic to 64 bit as explained here: