You're right, they have no right to be precious about coverage if they use the media for promotion but there's a point where 'bad press', for example reviewing a product Victoria has designed or is associated with and saying it's bad, and "personal abuse". Saying someone is a bad parent when that is to any reasonable person obviously not the case is abuse, not criticism.
Aside from anything else, it's not that the Mail etc. are saying this stuff out of any profoundly held belief; they're saying it because they know an item slagging off a celebrity will get lots of clicks. So it's not just abuse, it's cynical, self-serving abuse.
If you use your public profile for your own profit yes, you lose the right to complain about reasonable criticism and opinion expressed vaguely politely. But everybody on earth has the right to complain about being publicly abused, famous or not.
Being public does not mean 'anything goes'.