The behavior you observe comes with Marketing Cloud Connect, which is a managed package, meaning you cannot alter its behavior.
What MC Connect does is not controllable in the SOAP call either, at least I haven't found that documented anywhere. I would be greatly surprised if such an integral function would have been rewritten for an optional feature, and then it was forgotten to add that to either the function's or MC connect's documentation.
So you likely won't find a way to stop this from happening ex ante, unless you want to uninstall Marketing Cloud Connect.
Inside Sales Cloud, you can of course reset the field back to false after it was set to true, or use AMPscript updateSingleSalesforceObject function from SFMC to perform that action from a connected SFMC org. Both approaches however can only start ex post, so after logUnsubEvent has set the field to true.
Doing it in SFSC would be my preferred approach, as that is "agnostic", in that it also catches list-unsubscriptions, not just those LogUnsubEvents that you write yourself and possibly embed in some AMPScript.