I think parent was badly worded/thought out, but points to a real strategy. He said "charge for commercial use", which is definitely illegal, but what a lot of projects do is dual-license open source and proprietary. If you're a non-commercial project, you'll usually just use the open source version; if you're commercial, you'll want to be able to both link the project into a proprietary codebase and get support guarantees, so you pay for the proprietary license.
It is not "definitely illegal" to set license terms to whatever one wishes including restricting commercial use. The GPL hinges upon copyright to enforce it's provisions. Copying privileges are revoked for people who violate the license. This is a legal "hack" to get around the need to establish a contract or exchange funds. The same sort of rules can be applied for any other criteria such as the type of user. If I want to prohibit blue-eyed people from using my software I am free to do so. Dual licensing isn't a requirement to do this.
This is exactly what we ( http://linphone.org ) do: we have a GPLV2 license that's used in many many open source projects, and we have a specific license for commercial projects, for which we provide support and ability to not disclose the sources that our clients might modify.
This kind of way to function give enough money to employ 10 people, and also secured a lot of new developments (encryption, video, etc.). Disclaimer: I'm just an employee, not the founder.
I would advise him to switch to that kind of licensing, if he wishes to. There's no reason he shouldn't be able to live through his work.
One problem with that strategy is it forces you to use a licence with some sort of restriction (like copyleft) to make dual-licensing useful. You can of course dual-license a BSD or MIT licensed project but no one would use the "dual" part of the dual-license since the copyfree license allows them to do everything they need.
If you do this, the problem with dual-licensing a copyleft project is as follows:
-The copyleft license allows use in other projects with the same copyleft license
-The "dual" part allows use in proprietary licensed project (the stated enemy of the copyleft license)
-The ones left out are projects that use other copyleft licenses and copyfree licenses.
In my opinion dual-licensing copyleft licensed projects is a mockery of the purpose of copyleft.
All this is however irrelevant to the discussion about funding core FLOSS software. It doesn't matter the license, the challenges to funding such a project are the same.
