Whilst I fully agree that it seems unfair, my own experience as a dir of a listed Co. was that it was impractical (prospectuses, costs etc as pointed out by RedAce) and took too long.
The only way we could raise money (Co. was not going v well) was via a deeply discounted issue to soph investors.
Having said that, maybe a mandatory subsequent properly pro rated SPP to the non participating shareholders at the same price would be the way to go. For some reason we did not do this - it was a while back and maybe the rules were different back then.