I assume there is a programmatic reason for this, but having undo toggled off by default is dangerous. I have been burned in field calculations most specifically. My suggestion is to add a warning message. In ArcMap we were warned that calculations cannot be undo outside of an edit session. Since Pro does not use "edit sessions" the warning would read differently, but achieve the same effect. I work in 9-1-1 where mistakes can affect lives. We need safeguards in place to prevent them. Maybe add a warning to turn on undo? Users that do not want to see the warning could check a box to "do not show this warning in the future" Another option would be to set undo on as the default for all geoprocessing tools.