The individual heatsinks are cost-cutting and ease-of-manufacture measures.They are necessary because the mounting tabs are directly screwed to their heatsinks and so electrically connected. Mounting the finals using insulating? washer kits isolates them electrically so they can share a (larger/more efficient?) heatsink, but to keep thermal resistance low most such kits require a thermal paste, and that's extra work for Farhan's assember-ladies and quite likely an extra complexity in quality-control (-> extra cost). Upshot: grounded or no, a shared heatsink is possible but requires extra hardware and effort.
The above from long experience and usage in audio amps, similarly class-AB.
It's also possible that at RF ungrounded capacitive coupling may play other havoc :(
de ZL2DEX
