The "law of supply and demand" only applies in microeconomics, and only in a perfect world that does not, and can not, exist. It requires:
* Perfect competition
* Commodity non-distinguishable goods
* Perfect information flow
* No assymetric information
* No transaction costs
* Perfect mobility
* An infinite number of suppliers
* An infinite number of consumers
Break any one of these conditions, and it is an approximation, not a "law". Break a couple more, and you have complete market failure.
Raise it to the macroeconomic level (country wide or world wide), and it's completely useless and does not describe reality at all.
It is you who does not understand fundamental economics, whether capitalist or otherwise. (Oh, and a small hint: the US system hasn't been capitalist in a long, long time; since at least the 18th century, and quite possibly the 17th. It's actually financialist.)