I think the reason why valve is doing this is because people might buy a game at a higher price, either on Steam or another storefront, and then complain that it was cheaper on Steam or another storefront and start demanding refunds or demand that Valve reduce the game's price on steam.

What do you do then?

If you don't address it, you're automatically seen as the asshole even if it was the developer's choice.

You can give out refunds, which makes you look like the good guy, but that also looks bad to companies like Visa or PayPal (my understanding is that large numbers of refunds tend to look bad to payment processors, even if the refund was initiated from the company and not the consumer). Granted, Valve is a big enough company that they shouldn't have issues with that kinda thing, especially since they already offer refunds, but my understanding is that it still doesn't look good to payment processors and can make them upset.

You can ask the developer to reduce the price on steam, but what if the dev says no?

You can force the dev to reduce the price, but now you're even more of an asshole.

You can lower the cost on your storefront and cover the difference yourself, but now you're potentially losing money. That, if I'm not mistaken, is actually anti-competative from a legal standpoint.

You're kinda screwed if you're trying to be the good guy.

That's not even getting into how bad it looks if it's cheaper on steam than somewhere else when you have a marketshare as large as Valve's.