It's pretty simple. "Property" is a general term which doesn't need to have every particular example enumerated. Realized gains on the sale of property are subject to taxes in the US. Anything whatsoever that fits the definition of property, i.e. anything which can be owned or possessed is subject to taxation.

Here's the only legal document you need to know the IRS applied the law correctly: http://www.law.cornell.edu/uscode/text/26/1001