Because the NRL's rules applying to player contracts are different to those applying to club officials. The NRL requires a clearance from a player's former club to register them with a new one. This applies pretty much in all team sports at all levels. Club officials are aligned with business in general. It is not illegal to headhunt someone under contract, it happens in business every day, but if it is a breach of contract compensation or other measures might be imposed by courts and in most cases negotiation between the parties would take place to reach an amicable agreement.
You can't force someone to continue to work for you, even if they are under contract. THAT is illegal.
I am well aware of the rules, my point is the inconsistency in the ability to approach a contracted coach and offer them a contract but not a player. Let's not pretend a coach is just a 'club official', that's laughable and doesn't cut the mustard. A 'club official', would be like a trainer, a physio, a development officer. These roles are generally salaried, rather than contracted, hence the ability of these changing clubs being different and with much less difficulty
The reality is that coaches should be subjected to the same rules as player given that they play such an integral role to the onfield performances and development of the club. It is reprehensible but not surprising that the NRL management have allowed this loophole, given their total ineptitude. This is on the NRL, not the Panthers as they are just working within the parameters of the rules.