To begin with, what is or is not ok in a certain relationship should be decided between the partners, and they owe no justification to anyone else.
Forbidding all conversation/information seeking outside the relationship is another matter - that is a sign of an insecure control freak who will quite likely end up harming the sub.
I would not assign insecurity as a rule for forbidding talk to other Doms. It is after all a fact that there is a huge load of fakes - guys (or perhaps women as well) who just want to get off quickly and don´t care about the person they speak with, and who are not really interested in the BDSM dynamics.
It is possible her Dom is just protecting her in that regard.
I know of many subs who have restrictions in that area - though usually they are allowed to _talk_, but not to play with other Doms.
It has also been know to happen that a Dom lured a sub away from her Master, but honestly - if that happens, something was not all right with the first relationship to begin with. And it always takes _two_ people to make such a decision. Subs are not brainless items that can be "stolen" oder "taken" against their own will.
I myself have no restrictions regarding communications (except that I am not allowed to take anybody else´s collar), and I would not take well to something like that, either. However, I am married to my Dom in r/l, and he has known me long enough to judge me and know my mindset.
He knows very well that in some areas, I will not listen to well-meant advice, but need to make my own bad experiences to stay away from something.
Which happened on here as well, and which is the reason for my not going into private convo with any males except MasterStone anymore, who proved to be trustworthy in the extreme to me.