How is it theft that the people who do the work enjoy the benefits of that work? Why should the wealthy get 99% of the benefit of the people's labour? That's theft. Libertarians DO find theft abhorrent but the people who call themselves libertarians now are Prosperity Libertarians who are to libertarianism as Prosperity Christians are to Christianity. They are NOT libertarians.

Socialism is the belief that the product of labor belongs to the people creating it- Workers.

And not to the capital owners who contribute nothing to production.

In that frame of view, the Capitalists steal surplus value from the workers - The difference between wages and the value of product. And as such, taking back the means of production & the spoils of the capitalist parasites isn't theft, it's reclaiming of one's rightful property.