It's not like the price is fundamentally linked to scarcity, like v = f λ, or F = ma!
The seller makes a conscious decision (or programmes an algorithm) to decide what to sell at.
It's not like they'd be prosecuted for not increasing their prices as things get scarce, nor would the universe gasp as one of its laws Of Nature is broken...
Basically, a seller decides to increase their price over and above a reasonable margin in order to make even more money.
Now I have no problem with the idea of making a living, but taking the piss is a different matter!
Frinstance, in my shop we have a set margin; all prices are determined by this margin (we often round up, and sometimes down, to the nearest 99p or 49p). We don't say " ooh, calatheas are selling like hotcakes, let's double the price!". Our prices only change if the cost goes up (then not always), or if we want to get rid and will put it on sale, sometimes as low as cost or less! However, I've seen some of the products we sell at £10 more in big-chain garden centres - they're definitely taking the piddle as they'll buy at less than us as well.
Similarly, we have a few petrol stations locally - there's the usual Texacos, Shells and supermarket ones - they all have prices within a penny of each other. However, there's also a small independent. They're around 15p/l less. They made the decision to sell with a lower margin in order to give a better deal to locals and to sell more. As a result they're always busy and the extra revenue has enabled them to refurbish their previously dilapidated premises while the big player's forecourts are largely empty.
So I'd say that strict adherence to the "law" of supply and demand is just a wee bit immoral and most certainly doesn't serve the customer at all!