There are *so* many reasons for such behaviour: rudeness, taking people for granted, unawareness, forgetfulness, using other people or fakeness etc. There may even be genuine reasons (something happened, an emergency or distraction by 'real' life or their own crap). Goodness knows what goes on in people's heads or lives.

I agree that it is rude. As for unacceptable, it would depend on people and circumstances. For instance, if it is the same people, same pattern, or people only know you when they need you, then that speaks volumes and yes, it is unacceptable. IF it is a one-off or a behaviour that is unusual from someone, that would be different.