If you are genuinely asking, I can play Devil's advocate:
Because then they can set the price at 40 USD, making it more affordable, and possibly make back the difference with some (mostly) cosmetic premium content.
This is not so easy to argue for games that are sold at 70 USD, and premium content is much more tied to gameplay, and all the FOMO dark patterns are turned to max.
Why I think people are praising the helldivers2 monetization is that isn't the case. The "premium currency" is earnable in game and at a reasonable. I haven't bought any but still have the battlepass and a few of the premium armors.
You get it as part of the battlepass, and the gameplay loop guides you to the currency. You'll be looking for ammo or in game currency, and there also happens to be premium currency sometimes.
The battlepass not being timed and on a work at your own pace is great too.
It feels fair to me? Like the developer can still make a buck but not ruin the experience. I.e. the monetization lets people pay to instantly gratify if they want vs punish you for not spending.