Steam's terms of service mean devs/publishers can't promote a game as being free elsewhere. It's likely directed at preventing redirecting people to Epic or a sale at another store, but also by effect includes games like this. Either they put it for free on Steam or they sell it on Steam and can't mention free downloads from their website, and it's up to users to "shop around".

I get what you're saying, but I think if spending $10 on a game is something that would give someone pause, then the onus really is on them to search around first to see what the best deal is? People act like steam is a monopoly, but there is a lot of choice out there.