Many employers have clauses about owning any IP you produce while employed there, regardless of whether or not it involved company time or property. Whether said clauses are actually enforceable varies from location to location.

Aka "justice by bank balance and time".

At some point software engineering is going to have a union just as a legal defence fund.

Software engineers already have their own chambers of engineers in some states. No need for another trade organization.

The FSF doesn't want to get caught up in a dispute, and thus requires explicit disclaimers of work-for-hire interest from employers https://www.gnu.org/licenses/why-assign.en.html