Re: "but could for example, also be done through ads on a trusted website"
It's even - gasp! - irony
Nah. In most cases (sweeping generalisation) it's simply ignorance. We discussed a similar case here recently, "the proof is in the pudding". I'll bet that most people who use this phrase do it entirely innocently - not ironically - having heard it somewhere else and not realising that it's a corruption of a perfectly legitimate and explainable phrase.
In fact I'd go so far as to say that many, if not most, idioms have a rational explanation behind them. Even apparently madcap ones such as "raining cats and dogs" or the Welsh equivalent "hen wragedd â ffyn" = "old ladies with sticks". They bring to mind a particular sort of painful, lashing rain.