Roleplaying gets plot cookies. Rollplaying doesn't, because they're interested in stats.
It's not so much punishing Roll Play, but tailoring the experience to the player. A roleplayer will have a charcter who falls in lust/love, makes strong friendships and stumbles into awkward situations.
A roll player doesn't care as long as he hits stuff and does tons of damage. I don't, as a DM have much control over his choice of stats or feats, and s/he isn't interested in plot, so why throw cookies that only get ignored?
Because people offered cookies are more likely to try cookies. My brother was pretty distant from the roleplay aspects of my campaign I started months ago, because he hadn't figured out a lot about his Paladin other than he was a Paladin and had a bible and sword handed down from his parents. He was mostly blank beyond that.
I stuffed some cookies in his mouth and not long after he might as well been blue and furry.
This could be true, however, I have had only minor luck with people. I ended up running a game that ended up half ways revolving around the fact that a character with a specifically tailored plot refused to deal with it, and as a result, the rest of the party hated him and eventually tried to assassinate him.
Same player did the same thing to my husband. Was given a plot made just for him and spent two thirds of the campaign shoving it off onto us to work on, while he ignored it. He eventually died but not before he got full blasted about his neglecting to resolve all the issues he caused to the point where he was a walking natural disaster and noone wanted him anywhere around them.
I guess I just have stubborn players bytimes I haven`t run in a bit and that player isn`t in our group anymore, but I still hesitate. I put a lot of work into my games and I haven`t yet found a way to shake the habit.