I have my own reasons, but for instance a member that is logged in for weeks at a time, nearly indefinite thus far from time of registration. His activity shows 'viewing unknown page' makes me uncomfortable. Id like to just be able to kick or log the user out rather than ban. I dont know if theres something weird going on or what, but it is making me uncomfortable.
Its also known that there is an activity alert plugin for chrome that causes a user to be frequently viewing unknown page. It would kind of be like kicking a user off tapatalk and them no longer seeing what's up while they arent on your page. Basically reducing their real activity as well since they might not visit the site when not getting alerts that they have alerts. Think twice before you act.
There is a plugin for chrome. It retrieves a users alerts from your board and displays them on their computer even when they are not browsing your forum. It is known to show a user as viewing unknown page.
If the user cares that much to get alerts from your forum, then preventing this will likely reduce their actual activity within your forum. Something you likely don't actively attempt to accomplish. It would be the same as if you could make one tapatalk user stop getting alerts.