It's actually pretty standard practice, and it makes sense. If your phone gets stolen, and it has sensitive company data on it, then they need to ensure their data is wiped.
In the case of the Microsoft Company Portal app, and enrollment in InTune, a separate work enclave is created, at least today. The enclave is fully controlled by your company's InTune policies, but your personal enclave remains untouched. It's how MDM should be.
I don't think it makes sense at all. If they want to own my phone, they should give me one and not take the one that belongs to me and have all my personal stuff.
In the case of the Microsoft Company Portal app, and enrollment in InTune, a separate work enclave is created, at least today. The enclave is fully controlled by your company's InTune policies, but your personal enclave remains untouched. It's how MDM should be.