There are use cases where only symmetric encryption shall be used. Right now this can be done as long as no key has been created or imported. However, this feature is troublesome because users like to play and over short or long some keys will show up. And the users will curse because then they need uncheck all the boxes.
A configuration option to allow only symmetric encryption would be very helpful. In the best case the recipients tab would even be suppressed. But having this greyed out is probably easier to implement and reminds the user that there are better ways than symmetric encryption available.