Because it’s a dilution of their trademark. In trademark law, the law is very clear and states that the owner of a trademark has the legal duty to defend their trademark vigorously. To fail to do so will be interpreted as giving permission. And doing so weakens the ownership rights of the owner. Permissive use without license causes detriment to the trademark owner in subsequent infringement cases. So unless you fight hard on the first case, you may be much more vulnerable to other parties taking your trademark and using it for their own commercial purposes with a stronger defense against the original owner.