Have you played any of the Kingdom Rush games from Ironhide Games before? I seriously question whether you have when you call out the IAPs in this way.
ALL of the Kingdom Rush games have had large amounts of content that could be bought with IAP. EVERY LAST ONE OF THEM. In fact, most of them locked the majority of the 'optional' hero characters behind full paywalls (you could never unlock them through playing the game, no matter how much grinding you did, only through paying real money).
So yeah, nothing new here. Seriously, re-download one of the earlier versions and open up one of the shops. Very clear that they have always contained huge amounts of IAPs (it's not a new trend with this company at all). It's really my only qualm with the previous games, as they are well built little TD games.