Policy restrictions on using instruments for open-sourcing independent work?

Most companies give away laptops/mobile phones etc to employees while they join.Some of them ask people to sign specific legal documents to mention that , one would use them only for official instruments.Some prevent people from working on an open-source project ,labeling all such work as their intellectual property.This prevents people from officially contributing to open-source projects.This prevents people from learning and growth.Is there any solution for this?