The parties in this case were in divorce proceedings; they were, apparently, legally separated. There's a huge difference between being married and being legally separated; for instance, in a community property state (Michigan isn't one), property acquired after separation isn't community property.

What seems to have happened in this case is no different from what would happen if my babysitter borrowed our computer to access her email, and I snatched her password. Use of a shared computer to access your email does not surrender the account to everyone else who uses the computer.

It looks like the guy straight-up broke the law. Good for Michigan for actually enforcing it.