Segments and opportunities for campaigns are creted based on contacts.

If it could be created also for customers it can give to users more opportunities for example to create sales prices for special campaigns or contracts, it could be that customers already have price group but currently there is a special campaign for some of them (and they could be from different groups),but if users not using contacts, it will not be comfortable for them to create and support a price list for campaign.

I know, we can activte Allow Updating Defaults mode in price lists and do some workaround, but it might be an advantage to make campaigns more flexible