You are very lucky that no one has challenged the situation. A problem could arise if your check pilot stops someone before they even get off the ground and that person asks for the authority that they have to comment on another qualified pilots ability, what training they completed and to what standard they are certified.
DFC, I assume you have a drivers license. And still I do not allow you to drive in my car until you abide by my rules. Which may include you driving me around the block to demonstrate your driving skills and your ability to stick to my rules (no revs higher than 2500 rpm until the engine is thoroughly warm for instance). What's the (legal) problem with that? (And note that I'm not a licensed driving instructor or examiner. I therefore am not able to revoke your drivers license, but I can deny you the privilege of driving in my car.)
Now translate this to the aviation world. Why can the owner not specify his own rules, above and beyond what's legally required, for the hirer to follow?
The only "authority" a check pilot needs (even if he just holds a PPL) is a notice from the owner saying that he is authorized to conduct check flights, on behalf of the owner, for the purpose of determining whether a prospective or recurrent hirer meets the qualifications and flying ability/technique that the owner demands.
It is also only works in group situations where the checking pilot is willing to pay part of the costs of the flight that they have been asked to check a pilot on.
Only if the designated check pilot is listed as PIC. But we're trying to convince you that it's perfectly legal, and practical, for the prospective hirer to be PIC and as such pay for all the costs of the flight.