They are actually very clever about it. There is literally 0 alternative in the Rust world: this is what the "gpl competitive advantage" is about.
Most of us have our roots in the open source world, many (most?) of us learned coding because we were able to look at open source code, make changes to it, recompile and play with it.
We're trying to give back to the open source community and at the same time earn a living by asking the people who use Slint for proprietary apps (i.e. not giving back) to support us by purchasing a license.
All other so-called "free" licenses are just a way for corporations to co-opt the open source software community.