Here, see for yourself some of the tactics they use:
https://www.qt.io/download-open-source
> This dual-licensing model is based on the principal of quid pro quo – roughly meaning “something for something.”
> Simply put, this is how it works: In return for the value you receive from using Qt to create your application, you are expected to give back by contributing to Qt or buying Qt.
No you're not. You're expected to follow the license agreement, which requires neither of those things. You can't even argue an interpretation where GPL requires you to contribute back to Qt, and most of Qt is LGPL, which requires even less. The closest it gets is that if you patch Qt, you have to publish the changes. That's a pretty stark contrast there...
This entire page is full of fearmongering crap designed to scare people out of trying to use the open source version. It, of course, has some other very curious issues, like that giant blurry link which is actually an image with text, but what is perhaps most interesting to me personally is their other page about open source:
https://www.qt.io/licensing/open-source-lgpl-obligations
> The primary open-source license is the GNU Lesser General Public License v. 3 (“LGPL”). With the LGPL license option, you can use the essential libraries and some add-on libraries of Qt. This allows for keeping your application source code closed as long as all the requirements of LGPLv3 are met. More details are available below.
They go on to describe the restrictions. Some of them are fair, as they point out legitimate places where businesses may not like the terms, such as GPL 3's anti-TiVoization rules or their DRM agreement. However, some of the obligations are basically pre-fulfilled when you use the official SDK, which is what the previous download page was for. For example, if you use the official SDK, you will get dynamically-linked Qt. This is good enough to satisfy the relinking rules of LGPL. However, they never actually say this anywhere, since it would probably scare less people into a commercial license. Even with that grievance, this page is significantly more reasonable than the page that most people see, right before attempting to download the SDK (which, BTW, requires that you log in, even for the open source version. So build it yourself. It's easy.)
Due to their recent shenanigans with the KDE Free Qt agreement, I'm inclined to not believe a charitable interpretation of what's going on here. I want Qt to be successful and sustainable of course, but not like this.