The 'Tag in Mailchimp' button on a campaign activity's 'Target Contacts' list adds the entire original list of contacts to Mailchimp, regardless of what filters and/or searches are currently applied for the user viewing the list. This is unclear and improvements could be made to make it more obvious that it will add the original unfiltered list.
When you have a large database of customers all with specific requirements, having the column filters in an activity in order to drastically reduce your end 'target list' is vital. If it is for visual on screen only then the 'Tag in Mailchimp' is confusing as you only want to tag what is shown on the screen within Mailchimp in order to send those emails not the whole list. Having to go through the list and actually delete from the overall activity is very time consuming. Why not just 'Tag in Mailchimp' what's showing on the screen (with filters intact)?