Didn't MM ask privacy for Marius since he is not a royal ? What's the need to pose with him ?
Because he is her son?
Any mother will take pictures with her son from time to time, especially on holiday. That is very different to the paparazzi following him while he goes about his life as a private citizen, especially when he likely consented to these pictures being uploaded on his mother's account.
It is also a "damned if she doesn't damned if she doesn't" situation for MM. If she doesn't ever post about Marius, people will accuse her of favouring her royal children, but if she does post with/about Marius, she gets called a hypocrite because she asked for privacy for him.