Pretty sure ppl would not have bought as many boosts if they had to do it illegally. It was very tempting for example to go buy a hc Sylvanas boost on TN EU for 50k gold (yes, that cheap) instead of bothering with progress with your guild. When you have everything right in front of you and there are NO downsides, not even potential ones, no risks, why not do it. Buying illegally would be a risk and at least some would be deterred.
And tbh when blizz introduced that they also started designing limited time rewards for raids that you wouldn't be able to farm later even with 1% drop chance. So you feel tempted to do it just for that. Started with Garrosh heirlooms (that was the big boom) and continued with mounts for end hc raid. After Garrosh, ppl were selling boosts for everything, remember how ppl were buying boosts for just doing a CM even untimed for that one piece of loot in wod?