Why? Most (even salaried) full-time contracts require something like 38-40 hours per week. So long as what your staff are doing outside of those hours is not representing the company in a bad light, it should not be your concern. Trying to control what your staff do in their own time is toxic.
You can choose to hire double dippers then, when you start your own business. I am sure you will get a lot of quality work.
I really don’t think this view is out of the ordinary in the US. How many YV startups would allow someone to work for salary at a second job, if being paid a salary? I have seen many employment agreements, and it’s not very common.
It’s one extreme to own someone’s work output outside of work hours. It’s another to allow someone to collect two salaries.