What motivations do they have? Money, greed, the same thing that motivates other companies to do things to screw over customers. You have assumed I don't like Steam, but you are wrong. If I dislike Steam so much, why have I bought over 40 games from them? I like Steam, and that is why the introduction of this wallet worries me. Ask yourself this, why did they need to implement this Steam Wallet service if you can put just what you need in it for a purchase? The http://store.steampowered.com/subscriber_agreement/ (that thing you have to agree to, but never read) pretty much tells you of their motivation:
Any funds you put in your wallet can only be used to buy things from Steam, they can not be withdrawn. They want to get your money in their wallet so you have to use it at their store. Currently on the http://store.steampowered.com/steamaccount/addfunds/, you can only add funds of $5, $10, $25, $50 and $100. They may not call it "Steam points," but the concept is the same, to make you add funds to your wallet in fixed amounts that are more than what it costs to make a purchase so your leftover funds are stuck there. Yes, they currently still let you make purchases with credit/debit cards and PayPal, and I hope I am wrong but this could be the first step to gently get people onto a "point" type system.
You say "base your dislike of Steam on facts and opinions, NOT fallacies or irrational suppositions." I say don't let your unconditional love for Steam blind you into thinking that they would never do anything to "gyp" their customers.
That term there looks like it's for when your Steam wallet doesn't have enough in it, so it can automatically charge you for the defecit. It's like checking account overdraft protection.
You misinterpreted it to mean that Steam would money-grub.
Your terminology in the rest of the post also indicates you didn't take anything I said to heart.
My point still stands. And since you seem to be only interested in railing against Steam instead of thinking things through, I don't think any more discourse between us will be productive.