An excellent question. I think the thing is that for the prices that often show up in apps, maybe not? Like, for a $3 app I don't think I'm going to go visit some website, go through their flow, and get out my credit card if I'm only going to save $1. (Ok, I'd probably do it on principle at this point to stick it to Apple, but that's not the same thing.)
If I were going to shell out $100, I'd certainly go to the extra trouble to pay $70 instead, though.
The dangerous thing is that with all these mobile games that slowly siphon money from you, you might over time end up spending many tens or even hundreds of dollars (where 30% off might be a significant amount of money). But if each transaction is small, you might not think much of the difference of a dollar or so per transaction.