Okay, I'm a married woman, and at my previous employer, we were given desktops, which meant we had to use a personal laptop if we wanted to work from home, or check our email out of the office. Totally legal, not a problem.
So, what happens if my husband decides that my laptop is also his laptop, and uses it to access to company servers, accounts, and databases? I signed an NDA, he hasn't. Now he has access to intellectual property and trade secrets, he decides to go to a competitor and sell them for the highest price.
Fortunately, the ECPA exists, so even if my security practices were as terrible as the above scenario implies, I'd still have some legal protections.
If I've told him "this is my computer, you're not allowed to access it" and he did, that would violate the ECPA. If I protected it with an easy-to-guess password and he guessed it, that would violate the ECPA. If I had an unprotected computer, and let him use it, but he installed a keylogger to get work passwords, that would violate the ECPA. If I let him access my machine and he uses auto-fill stored passwords in a browser to log into my work email, that's also an ECPA violation.
Not only does this apply to husbands and spouses, but also parents and children.
In addition, the separate/community property laws vary from state to state, and most states allow for common-sense separate property laws within marriage. In most states, my spouse can't use a computer given only to me, or a computer I had before we got married, or a computer I purchased with my own separate assets (what constitutes a "separate asset" is a huge discussion, but you get the idea). You can make a written agreement that "this is separate property" and even if you didn't do that, it would likely be a strong argument in court that an unwritten "separate property" agreement exists for a particular machine, in certain circumstances.
"Just because your wife buys it, it's still yours" is absolutely not an ironclad rule.