It's usually to offset cost of getting crafting recipie. People tend to think "this item costs N on AH, but it's mats cost only half of it, I'll outsmart those greedy crafters and get mats, and get it crafted for free", while forgetting that all those crafters still want to earn back money used on making it possible to craft this something, which is why they don't let you 'outsmart' them and still charge their fee. You don't like it, you might always look for other crafters, or just go ahead and learn it yourself. But it'll still be cheper just to pay what crafter asks. Traditions in real world vary by region, and in MMOs they vary by games. It does get troublesome in new games without established "traditions" on it's own, and "tips" are perfect example of that.