Hi, our admin is sick again, so he probably won't mind if I answer your question =)
And the following is just my opinion.
If a developer creates a project for himself or for a commercial purpose, I think it's necessary to pay all of those whose code, models, sounds, graphics were used in this project.
I think it's just fair. If someone helped you achieve your goal - you should thank him, support him financially with his further developments and improvements.
Assets on this site should be seen as introductory versions, which help in order to unambiguously determine the choice of asset. The essence of the problem is that you need to spend a lot of money to find solutions "for yourself", which you will actually use in your project. Some assets are outright fraud, some are in a permanent beta stage, some don't solve the problems you expect from them.
But if the asset fits your project - buy it, if it possible, support the developer. It's the right thing to do.