> why should this be the job of acts as taggable?

We are using Rails 4 with the protected_attributes gem (for backwards compatibility to ease the migration to strong parameters).  Acts as taggable is checking ```defined?(ActiveModel::MassAssignmentSecurity)``` but in our scenario protected_attributes has not yet been required, so ```ActiveModel::MassAssignmentSecurity``` is not yet defined and attr_accessible is not called.  Later, setting tags fails because no attributes are accessible.

> why should it be in this file?

This is where ```ActiveModel::MassAssignmentSecurity``` is referenced