The activation process sets (or in this case, resets) the password when in the default install. The flow is that the user registers, then the admin activates, that is when the login credentials are sent. Since WP encrypts passwords, and we don't want to be storing unencrypted passwords, we need to send it at that point.
So, in bulk activation, the users that you are activating are being sent their login credentials, which would include a new password.
The way around this is to create a password field as part of your registration form in the WP-Members fields tab. Create this with the option name of "password".
At this point, there's two directions you can go.
(1) If you still want to have the default way the plugin operates, then you are just creating this password field for the bulk activation process with the imported users. When there is a user chosen password involved, then the plugin does not set a random password to be sent. (You will want to take the password shortcode out of the email that is sent as it would just be empty.)
Note: once you've got your imported folks activated that way, then you could take the password field out of the registration form and have new registrants be sent a password upon activation (which they of course can update to whatever they want).
(2) The second possibility is that perhaps you want to stay with users choosing their own passwords at registration. That's fine, just remember to update the emails that are sent accordingly - If you want to send login credentials in this mode, you need to do it with the initial registration email, since the password has not yet been encrypted. Be sure to take the password out of the user approved email as the encrypted password is not set, so it's just a blank field in that case.
Hope that makes sense and things work out for you. It's really not as complicated as it sounds - it's just that there are a lot of options beyond the default installation, so there's a lot of "if you do this, then this happens..." So if anything needs clarification, let me know.