That's definitely a good question!
There are transaction fees on Hive, they just work differently than most of other blockchains. You could definitely pay block producers (BP) with inflation on the second layer too. But, then you have to find a utility for these tokens otherwise your BPs are "working" for free (or for tokens that are worth nothing). Also, having such tokens will definitely trigger speculation, which I would really like to avoid. Let's say a transaction costs $0.00001, then no matter how the price of Hive goes, you'll still pay $0.00001 for your transactions. I'm not saying that the users have to pay for everything, developers can pay for the fees (or part of them). When you host your app on AWS, you have to pay for the usage, same goes for blockchains. Also, when you let people run arbitrary code on your platform, you want to have a way to limit them, otherwise they'll start eat up all the ressources available.