The accounting definition is all that matters...
...but since I do that all day long, I'm a little biased.
There's also a difference between salaried and hourly wages, legally. A salaried person is going to get their pay regardless of working 40 hours or 100 hours. There is also a difference between exempt and non-exempt for salaried positions in paying overtime. Hourly employees get payed for however many hours they work. Period, plus overtime for hours worked over 40 in a week.
For a guitar builder, unless you have a constant stream of revenue from the business, you can't pay yourself a salary. So you can really only pay yourself an hourly wage - at best, if monies come in. And then wages can be payed out, taxes paid etc.
The partners in the law firm are not payed a salary, because they share the profits by contract and agreement. If your law firm made no profit in a month due to your substandard hourly billing for services

and since you're not technically a salaried employee, you would not get paid, I'm assuming.
So for Kelby's argument...
It is similar for the guitar builder, in that if they don't make sales, they don't get paid. The reality of a sole-proprietorship is that the IRS is going to charge taxes based upon what income you have minus your expenses (unless you pay yourself a salary and have taxes taken out). After that, you can figure out whether you made $50/hr or $0.50/hr. More than likely the latter. How you define the costs on your books plays into all that. Pay yourself a salary or an hourly wage, or it's revenue vs expenses and you're keeping the profit.
Dave may have a different take on it....