There aren't many. Bob Bittner owns 250 w WJIB 740 in Boston and some stations in Maine that are classified as commercial but he doesn't run ads. He plays standards and fundraises from his listeners. Doesn't streamcast--his stations are considered commercial even he doesn't run ads, so fees are priced at comm. station rates. Hobby? Break even? Maybe, but he's the exception that proves the rule, as radio is a business.
Radio is a business in a constant state of change. The fact that things weren't done a certain way in the past is not evidence that they won't be done a new way in the future. There was a time, until recently, when running an AM radio station was a good business to be in. An entrepreneur looking for a good business to invest in might well consider an AM radio station as a smart investment.
But times have changed. A standalone AM, with no FM attached, is no longer the kind of business that a sensible person would want to invest in if judged solely on its merits as a means of generating a good return on investment. To invest in a standalone AM radio station in 2014, someone would have to have some personal desire to operate a radio station for a reason other than making the best return on investment. That means someone like the fictional Charles Foster Kane, who operated a newspaper that lost money because he enjoyed it, and had enough money that he could afford to indulge in his personal whims.
Running a radio station as a hobby doesn't mean that it isn't run to generate at least some revenue. Radio is a business, but there are many businesses today that were once profitable enterprises that are now operated as hobbies. Take the movie theater business as an example. Once upon a time, all movie theaters were single-screen operations. Then, the multiplex cinemas came a long and turned the single-screen theater into the movie business equivalent of AM radio. There are still big, profitable businesses operating multiplexes (though they are facing an uncertain future as well now). But many of the few single-screen movie theaters still operating are "hobby" operations. Not all of them, but chances are that when you hear of a single-screen movie theater reopening under a new owner, that new owner is someone who loves movies and is willing to spend his own money operating the old-school movie palace on a business plan of just breaking even.
That's not to say that there aren't entrepreneurs with bad judgement who might want to buy a standalone AM station under the deluded idea that they can compete head-to-head against the major chain broadcasters. P. T. Barnum was right about the birth rate of suckers. But what's more likely is that someone who is retired from broadcasting who has deep enough pockets to buy a standalone AM station so that he can revive a musical genre that no other station is playing, and operate it on a shoestring, will decide to keep himself busy operating that old, standalone AM station.
Personally, I'd like to see someone buy 1250, change the calls back to WCAE, and program nothing but re-broadcasts of old radio shows from the Golden Age of radio, when stations like WCAE, WWSW, WJAS, and KDKA dominated all family home entertainment in Pittsburgh, especially if they'd stream it online. A whole new generation of people could learn the answer to, "Who knows what evil lurks in the hearts of men?"