Should contributing code to open source software require professional licensure?

As far as I know most (all?) open source and free software licenses include terms, that explicitly states, that there is no warranty. So I think maybe a license there wouldn't be required. It is an interesting question though.

But many people are paid by their companies to work on OSS.

Most commercial software doesn’t have a warranty either.

In that case I would say, since they are getting paid for their work by the company, they are in a different position than someone developing FOSS on their own private time.

I think a lot of commercial software that is not open source or free software, doesn't have licenses in the same sense. They are proprietary and they might have an EULA, that prohibits you from reverse engineering or something like that, or that declares the no warranty. But not licenses like for example GPL or MIT license. Such a license would be useless for proprietary software projects, because the user isn't supposed to ever get the code.