Why does selecting one (or more) tag options in a form created using webform-civicrm add *all* tag options to the user record?
So here's my answer, and solution. I started again from scratch. The steps I followed are:


deleted the Tags and Groups components from the webform
disabled Tags and Groups in the CiviCRM tab for Contact 2
re-enabled Tags and Groups in the CiviCRM tab for Contact 2
added Tags and Groups components back to the webform.


Then, under the Tags component settings in the webform, I selected:


Static Options (Fully configurable)
Checked "Multiple (Check this option if the user should be allowed to choose multiple values.)"
Unchecked Enabled checkbox at top of Options, the default, un-enabling all Tags as options
Enabled only the handful of Tags I want on the form
Clicked Save Component at the bottom


I then tested selecting Tags on the form, submitted the form, and only the three tags selected appear in the contact's record.