I also think, this isn’t that big of an issue. I this behavior is not wanted, just turn of the “shared organization” option.
But I guess what @scheeles tries to say is, if a company automatically add users with an E-Mail-address ending at @my-domain.com, than someone can go ahead an register with name@my-domain.com and can see all the tickets without verification if the user has access to the E-Mail-Address.
A verification E-Mail would make sure that the user at least has access to an E-Mail-Adress ending at @my-domain.com
But still you are not sure, if the colleague/employee should see all tickets. This probably depends on the type of clients you’re serving.