Some of that is guys adding it to their Camaro (all first gen), but specifically with the Z, there is a wide spread belief that all Z cars came with spoilers and the cowl hood. I bet if you asked people and owners at a all around car show, most people wouldn't even know that the cowl hood was only an option on the 69. Just like people think all Z cars came with the console gauges.
You could have a very original low option Z, next to a highly optioned clone, and most people would flock to the clone, and not be aware that it was a creation, but discount the original and think it is just wearing the emblems, because it doesn't have the eye catching options.( just my 2 cents, from what I have observed)