Scalpers are in fact adding a service.

To someone perhaps, but not to Valve. They set their prices where they set them for a reason, and easily could have set them higher if they wanted to provide the same 'service', I doubt the cost of an extra byte of bandwidth for the http responses for an extra digit in the price was the reason for making the price lower.

So Valve has no incentive not to fuck them over by misleading them about supply, and apparently at least some incentive to do so.