According to what Hilmar et al said during an interview on Talking in Stations, one of their goals is to make it easier for new players.
Ventures are mostly used by new players (for mining anyway), so coding trigs to fry these noob frigates in hisec belts seems to be counterproductive to their goal of a less harrowing new player experience.
IMO, if a new player- without the situational awareness that capsuleers who stick around eventually/hopefully develop- is made to feel that even the NPCs are overpowered, they may quit before learning how interesting EVE can truly be. Ship replacement cost isn’t the only factor.