Marketo and Employee Data Protection Regulations

Depends on which exact laws -- and which exact lawyers.

Assuming the content of the newsletter itself does not reveal employee data (i.e. you're not loading anything about them into Marketo) then it probably comes down to tracking.  I've never heard of a concern about email addresses alone. (After all, any user of a SaaS app is "storing" their email address in that app. But lawyers can come up with anything, even if other lawyers would eyeroll × Infinity.)

But if you don't turn off link tracking + Munchkin for these sends, you would be sending data about the recipient's web activities to Marketo. And that would also be true of their home computers. Definitely needs to be considered.

Of course there are reasons to not  use Marketo for corporate comms, well outside of privacy concerns. To guarantee delivery you need to whitelist your emails somehow. And whitelisting is not recommended because (1) it disguises deliverability problems for your "real" Marketo emails and (2) it can allow spam or malware into your company, which is... bad.