THEY MUST BE PUNISHED!

A mixture of enthusiasm, happiness, and disbelief struck Duke Galan upon hearing what Robin said.

After seeing what Robin could do, especially after the spirit Revitalizing pills he had sold at the auction, he realized that he could not force him to do anything

Because forcing Robin in something would be putting him in a box. 

if he was stupid and forced him to only make fire talismans from the beginning, he wouldn't have invented those pills or the darkness talismans which now he desires even more than the fire talismans! 

keeping him alive, making a good relationship with him, and hoping he would continue dealing with him, was the best and only choice

In his mindset: he needed Robin's future achievements and could not do anything to him, and Robin needed him now and that's why he will never hand him the real techniques or the methods to do all the work because then he will be less valuable...