it's not what 'Steam' allows, its what we allow
In all honesty, it is only because I came to the forum that I see you only ("technically") allow 1 PC per user, but do you really think that most Steam users would bother to read the EULA? A normal Steam user will probably see your product on the store and buy and install it if they like it. And if they have several machines, then they will obviously use it on them all, because that's how they expect software on Steam to work. Unless you go around giving fines to all your Steam users and cause an outrage, or make it vary clear that they can only use it on one system before they buy the product and impossible for them to use it in more then one PC, I don't see many thinking twice before doing so. And how would you even solve the problem of buying more than one license on Steam? As far as I know, you can't buy a product more than once on Steam, unless it's a gift for someone else. The only thing that comes to mind, is a tradable "in-game" item that allows the owner an additional system, but I don't know if software have such an option.
I own 2 licenses through SD for 2 PCs, but I didn't know you sold it through Steam. Someone I know later bought it through Steam and uses it on his PC and laptop, which made me feel stupid at the time
EDIT: Food for though, If hypothetically a developer on your side made the users of Fences into a botnet and attacked some server, are the users responsible for that, or the developer? My point is, unless you can enforce "1 buy, 1 system" through Steam and make it very VERY clear, I don't think that you can blame the user for "braking" that, blame Steam instead for the wrong way of distribution, or don't use Steam at all! But, I am no expert in the subject, so that's just my opinion... ¯\_(ツ)_/¯