Dollarama does not let customers use the bathroom because they're often located in stockroom, and the store's liability insurance often only covers customers in the main shopping area. So they're not telling you no because the employees are unempathetic, but because they are expressly forbidden from letting customers in the back room. While I understand that it most be frustrating for someone that has to go, if someone was injured in the backroom they could (and probably would) sue the ass off the company.