i discussed this at length with our physio, who was completely unable to come up with any actual reason.
they have similar issues with the door frame bouncer things.
dd1 and ds1 both used walkers (ds1 was born in canada - i had no idea at all it was illegal here and am now completely curious how i managed to buy one and use it ) and they both walked at the average/ early point (14 mos and 11 mos respectively) ds1 was obsessed with the door jumper as well. 
dd2 has cerebral palsy, and the walker thing came up because she couldn't walk independently. or sit. or stand, or whatever. and the nhs have a variety of different walker type affairs/ gait trainers that are available. (most rely on arms rather than crotch to hold you up, but by no means all)
so we had a chat about walkers and jumpers. and she did the whole aghast thing. and then came back a couple of weeks later to ask if she could borrow a few of our things to try with another patient. as long as the child has their feet flat on the floor and isn't dangling and tiptoeing...
all things in moderation. if you intend to suspend your child by his crotch for hours every day poised above stairs, then i willingly admire the nhs attempt to dissuade you (actually i don't, because the whole nanny state thing sticks in my craw). if you want to use one for an hour or so and your child enjoys it, and you employ the usual health and safety safeguards within the home (including remembering the child can now reach much higher lol!!!) then fill your boots.
dd2 now walks independently btw.
cp, walkers and all. 