Those in power define the rules and define things like 'theft.' Theft in the traditional sense is taking physical tangible resources that aren't yours. When we move to intangibles like time, businesses have defined all the rules around theft, not people.
In practice, the situation is rather gray. Employers will virtually never call the police in a case of theft (or "theft")--they'll simply fire the person involved. Likewise, most employees won't do much if they're stolen (or "stolen") from by employers, they'll just quit.
We're not even consistent in the ways that we think about the topic. There has been talk of a "rent strike" during the pandemic, which amounts to stealing resources from one's landlord (who might be "rich" or might be quite "poor"). Few people would go along with the idea of a "grocery strike", in which those who need food but cannot pay simply shoplift from their local store. Somehow the former sounds more okay than the latter, even though the former would typically involve theft on a much larger scale.
And of course, most of us posting here are "stealing" from our employers in some sense. The better employers typically realize that they're better off looking the other way.
If a task normally takes 10 minutes, and an employee completes it in five, is that a donation of effort?
Is it theft to titrate the productivity of your labor to fit the rate of pay you receive for it? Employees do not get paid more for effort that exceeds par.
If I get paid $7/hour, I can easily reduce my productivity until one hour of my labor produces $21 for the employer. Or maybe I work two hours to produce $160 and slack off for six.
Two consecutive generations of not being rewarded for contributing additional effort for the benefit of the company has taken its toll, culturally. Nobody is willing to uphold a string work ethic for an unethical employer.
The employers burned through all their credit with labor, and are trying to refinance by redefining all the rules. It won't work. It's time for them to pay up.
Only two? Someone skipped history class. :-)
Prior to that point, increases in worker productivity positively correlate (at least weakly) with improvements in the purchasing power of worker wages. Afterward, working class purchasing power went flat or dropped even as their productivity shot through the roof. The owner class acquired the ability to capture everything, and they chose not to let it trickle down, by the terminology of the time. They didn't spend it. They loaned it or invested it, such that it never fully left their control.
After the abolition of slavery and advent of industrialization, labor movements have occasionally been able to claw back more of the workers' share of trade in the specialist economy, and then keep more equitable sharing of revenues going for a while afterwards, but the working class hasn't caught any breaks for a solid 40 years now, and unions are now relatively weak, compared to their historic peaks.
Prior generations got peanuts for their extra efforts, but they did at least get something.
In my experience, "1/2" is too much (depending on how you define stealing). But it was quite common for both employees to steal from their employer, and for employers to steal (wages) from the employees.
It was also quite common for employees to simply walk off when they felt they'd had enough.