Or they make the barriers slightly shorter so that motorcycles can park for free.

Which they do, recognizing the fact that if there are a few motorcycles parked in your lot, it makes the whole lot look cooler.

Interestingly, I find that on my bicycle I can trigger the sensors in roads (which in the UK seem to be marked by a line of tar, where the sensor has been cut in and sealed up) by angling my front wheel directly along the line - my belief is that it gives a pretty strong signal because the rim of the wheel is a lot closer than a car would be even though they are a heavier chunk of metal (inverse square relationship). In fact my impression at one place I worked was that the sensor gave a stronger signal doing this as the barrier to get out went up quicker for me than it did for cars. I have extrapolated this belief and also believe that the sensors that detect vehicles before traffic lights also give a stronger signal when the rim is close by and the lights choose to turn my way quicker as they think I'm a bus. I can't really verify this though, because I only ever travel by bicycle.

Anyway, I guess motorcycles are not quite as maneuverable as a bicycle, so would find it harder to trigger the sensors in this deliberate way.

The trigger for lights in the states is electromagnetic. A lot of bikes don't produce enough to trigger so placing a magnet on the frame below the bike helps get you through the lights quicker.