You're ALWAYS guaranteed what you asked to purchase. You're guaranteed to be given a set number of in game items that you agreed to purchase. You're not losing value (what if a slot machine gave a sticker?). You're getting what you purchased, which are in-game items. People are just confusing the chance of their personal evaluation of that item as a "risk of money".
Think of it like this. You go into a grocery store and ask to buy four apples. The only catch is the store gets to pick what four apples you get. You might get an apple type you don't like, but you always get four apples. Your personal opinion of whether or not you like granny smith or washington golden does nothing to devalue the apple, which would be required in order for you to "lose money".
To put it even simpler, you are not gambling when you buy a pack of starbursts just because there's more of a flavor you don't like. A value has been assigned to these items/fruit/candy. You exchange money equal to that value and receive due return. The only thing at risk is your personal taste.
Think of it like this. You go into a grocery store and ask to buy four apples. The only catch is the store gets to pick what four apples you get. You might get an apple type you don't like, but you always get four apples. Your personal opinion of whether or not you like granny smith or washington golden does nothing to devalue the apple, which would be required in order for you to "lose money".
To put it even simpler, you are not gambling when you buy a pack of starbursts just because there's more of a flavor you don't like. A value has been assigned to these items/fruit/candy. You exchange money equal to that value and receive due return. The only thing at risk is your personal taste.