L298 high side driver: how does this actually work?
I think you'll probably find your answer in the L293 data sheet - it's the lower-power version of the 298 (without the heatsink). Here's what the 293 output stage looks like: -



The 293 (and I strongly suspect the same for the 298) has a PNP transistor that is much more conveniently driven from a low side logic signal.

I'm fairly sure the two devices work internally the same way - they have pretty-much exactly the same poor output specification. The diagram shown in the 298 spec "hints" at the limitations inherent in this type of H bridge because of the use of NPN transistors at the "top" and darlington transistors at the "bottom". See also this for alternative devices using MOSFETs.