That's an assumption. Following that assumption, you would need to ask your employer if he agrees with any tiny contribution you do to any open source project out there. Which doesn't scale. What the employment contract does is own the contribution precisely because it doesn't make any sense to ask permission everytime an employee writes anything in his own time.
You're saying it's "an assumption" that only the copyright holder can dictate the license? That's not an assumption. That's the whole point of copyright. The owner of the copyright determines the terms under which others can use the copyright.
Consider this: every open source project that has wanted to relicense but did not explicitly require a copyright assignment, had to talk to every single contributor to get their permission to relicense.
> Following that assumption, you would need to ask your employer if he agrees with any tiny contribution you do to any open source project out there. Which doesn't scale.
This gets into not what is legal but what is reasonable and sensible.
If you write 20 lines of code using your work computer on work time and don't have prior authorization to give that code to an open source project, you technically have to get your employer's permission. Realistically, few employers would frown upon that.
There are still companies out there that are not open source friendly. If you work for one of those, you'll find that you actually do need to get permission before contributing code.
I should note that I am taking the US perspective on this. Different countries have different views of copyright, and as noted in the original post different US states have different views on employment contracts.
IANAL, but I have managed and worked closely with some reasonably large open source projects and spent more than my fair share looking into licensing terms.
