You are only a dive instructor when you are instructing

You are only a dive instructor when you are instructing, other duties in a dive business may include working as a divemaster, dive guide, boat skipper or behind the counter but you are only an instructor when you are instructing. 

Naturally, it is a more efficient use of resources to have dive instructors performing those roles, as a dive centre manager, you have the ability to engage in job rotation to prevent people burning out. If this principle applies to dive centre staff, it certainly applies to customers.

So why is it when divers show up at a dive centre as paying customers they need to let all and sundry know they are dive instructors? At this point they are paying customers. I never gave discounts to instructors who dived with us, why would anyone? 

You are not getting paid so you are neither a dive master nor instructor at this stage. Would you let this person skipper the boat - not likely? Imagine the problems that would arise if an accident occurred, even though you have signed a liability release and possibly code of practice - the dive centre is liable.

When you show your rescue diver card, it tells the facility that you have done an AOW course, now you are not depth limited to 18 metres and you are capable of self rescue, now they should leave you alone to a degree. Any competent staff can work out your actual diving ability by the way you prepare, gear up and enter the water - that's all you need. 

When they ask about deepest dive during sign in, it is 40 metres as a PADI rescue diver, you have done 100+ dives and your last dive was you actual last dive, you have been honest and have not overstated your experience or ability. If they are not paying you to instruct, then you are not an instructor for this facility - it's as easy as that.