I don't understand this, although I've read something similar many times before; it seems incoherent. All value comes ultimately from labor, I would agree, but if I'm using tools to do work, a portion of the value of my work is attributable to the work done by the creators of the tools I'm using. Not me. It makes a big difference if I have a computer, or not; if I have a backhoe or a shovel. So this idea that capitalist profit is theft from workers has never made any sense to me. If the capital was stolen, then the proposition is tautological, but otherwise you are begging the question of what was actually stolen. I don't see a political or moral revelation in the claim that my employer is necessarily stealing from me. How can the assumption someone else is stealing labor justify you or me stealing labor?

P.S. If profit is illegitimate, then is interest also? I tend to assume that there's no moral or political difference between stocks and bonds, but for all I know other people may think differently.

One possibility from my point of view is that people look at profit as something extra, hence illegitimate, but the way I would look at it is that it is a deferred payment for work done, equivalent to interest. If you store seeds, then it is right and fair and necessary that in a well functioning society, you be paid for preserving them as well as producing them. Society can't function if people are paid only for producing things and not for preserving them, or if they are paid for preserving things before they do it. That's the most reduced scenario I can make up about why interest/profits seem legitimate to me.