One thing is for sure. BL's customer service sucks. I would have done the right thing morally, returning the lenses, however, if I was given an offer of compensation for my good deed, then it was taken away AFTER I did the good deed, then I would be pissed too.
In any case, even if the OP was an a$$ and demanded a reward for doing the correct thing, BL should be HAPPY to offer him a reward for returning the items. Two free rentals is a small price to pay for the return of these lenses. BL went to the EXTREME, they rejected the reward agreement, and then they banned him. Even if they gave him the reward and then banned him, that would have been fine. In this society, lots of people are happy to pay a reward for the return of lost items. Lots of people happily pay rewards for dogs, equipment, anything that someone wants back. OP FOUND something that BL did not even know that they lost.
Instead of coming here and making things right, Markol came and just stuck by his words. Even if the OP was an a-hole and demanded the 2 free rentals, Markol should oblige. It does not actually cost them a huge amount in funds to give him 2 free rentals. BL is running a business. This is a business world. Nothing is done for free. Markol expects him to 'return the lenses for free', but do not want to adequately compensate him for his efforts. If Markol is sticking by his business policy of ALWAYS making money, why should the OP do FREE work for Markol? Shouldn't he get paid as well?
Imagine the HUGE positive PR impact it would have if the OP came here to share his wonderful story about how he found the lenses, returned them, and BL gave him 2 free rentals for his troubles. Instead, we have a huge negative PR impact, and BL is still sticking to their ways.
BL needs to hire someone like Helen from Adorama to work for them. They need some lessons in customer service and positive PR.
All moral issues aside, business is business. Plain and simple. Pay the man for the work he did. Compensate him, call it a day, and move on. It was YOUR mistake. Not his.