Harrier has a canopy bow...canopy doesn't separate from a/c during ejection, instead it shatters. Can't eject on the ground with canopy open...well you can, but you're going to leave your legs in the jet.
I imagine the bow is there to provide structural integrity. Looks like a bulbous canopy anyway, at high airspeeds (Mach 1+) there's probably a lot of pressure on the canopy right there at the bow. IIRC, the JSF canopy stays attached during ejection too...don't want it going down the lift fan in a hover/slow flight with the fan doors open.
I imagine a/c with canopies that separate completely rely on forward airspeeds to help separate the canopy so the pilot doesn't pull a Goose. In an a/c that may have very little or no forward speed, it would take too much propellant and/or time to generate that separation.