Why do I have to enter a recipient after selecting users in the VBO send emails operation?
VBO does not provide the "send email" function. It is provided by core actions and it really doesn't know the context that it is being used in. Therefore, you need to insert a token in the "Recipient" field as mentioned in the help text just below the field e.g. user:mail , node:author:mail etc.  I can't tell for sure what the base table  your view is using based on your screenshot but it looks like you can use the token [user:mail].

If you would like to have this info prepopulated you need to use the views send module. It used to support VBO but it now provides it's own UI. It's a pain if you want to combine it with a VBO view as you get double checkboxes etc...

See http://drupal.org/node/575980 for more info