These adults knew fully the value of what he was doing for them. He didn't have to tell them about the importance of his classes before they knelt before him and thanked him profusely.
The interesting thing was that no one came to make a fuss when he began teaching the advanced mana breathing technique for free. Not a single person came to dissuade him or disrupt his class.
There were still a lot of dojo masters who didn't like what he was doing, but they didn't have the guts to complain about it. They had sent a second mortal coil cultivator to pressure him, yet the cultivator failed and also disappeared.
It is true that Black Mantis was a subpar second mortal coil cultivator. But that wasn't a bad thing because that's the main reason why they could afford him.
Black Mantis was unscrupulous and willing to bully a small boy for money. He also wasn't expensive, so they hired him to do the job.