The concept of private property advocated by America's Founders is mostly based on Lockean ideas of "labor mixed with natural resources creates property". But as Benjamin Tucker pointed out, that doesn't justify absentee ownership (either as landlord or as employer), since you're no longer there actually mixing your labor. And landlords and employers are exactly the roles who Proudhon and other anarchists see as illegitimate, not the small farmer who tills his own land.
Of course, as large absentee owners themselves, the Founders were only too happy to let that consequence of Lockean theory slide, just as they were able to write "all men are created equal" while keeping men in slavery. But that merely shows that Upton Sinclair's quote, cliché as it may have become, still holds an inescapable truth.