Activate subscriber without changing password

If you allow users to choose their own password at registration (creating a password field in the Fields tab for the plugin) and that fields option name is “password” (see the very end of http://rocketgeek.com/plugins/wp-members/users-guide/registration/choosing-fields/ for notes on adding a password field), then activating a user will not change their password.

If you have added a field for the user to choose a password at registration and it is changed when you activate the user, then you probably have given the field an option name other than “password”, in which case you need to change that.