I'm not sure that is true. The IRS doesn't engage in arguments as to what constitutes a "Professional" in any field. All they care about is their "pound of flesh"
One can make $100K in your regular career, and have a side business or LLC etc. ... the IRS wants its' cut of your income regardless if it is only $750 on the side.
Business involves taxes on profits from any enterprise after "cost of doing business" expenses deductions ... however you have to show a profit in one of three years, or you forfeit the business deduction status (i.e., it is a hobby in laymen's terms).
If you make $750 selling your hobby photography, and do not claim any business expenses, it is still income and subject to income tax.
- Marc