A lot of people do not care about licenses at all. No matter what you list as the license, they will do so if they can download and run it. Over the years, I have tried this multiple times where I provide the same code under a copyleft (free) license and commercial or permissive license (paid). Not a single time I've had anyone come to me and pay me to make it around the copyleft license. Any time someone has purchased the other option, it has been because of potential support options, etc.
So in EmailEngine, I added a license check. The code is always the same, but if you do not provide the license key (you need to pay to get that key), EmailEngine does not work correctly. Specifically, it stops processing email accounts after 15 minutes of runtime. You have to restart the application to get it functioning again. You would have to modify the source code to overcome this, but EmailEngine is distributed as a binary. To edit that code, you have to download it from Github, modify it and figure out how to run it. It's all straightforward, so it's not an obstacle for anyone with enough dedication. For others, it is easier to pay for the license key than to figure out how to circumvent it.
So in EmailEngine, I added a license check. The code is always the same, but if you do not provide the license key (you need to pay to get that key), EmailEngine does not work correctly. Specifically, it stops processing email accounts after 15 minutes of runtime. You have to restart the application to get it functioning again. You would have to modify the source code to overcome this, but EmailEngine is distributed as a binary. To edit that code, you have to download it from Github, modify it and figure out how to run it. It's all straightforward, so it's not an obstacle for anyone with enough dedication. For others, it is easier to pay for the license key than to figure out how to circumvent it.