There are two main models -- we could label them the "Redhat" and "MySQL / Qt" models.
The Redhat model establishes a brand, often the original source for the product / project and uses that to market their expertise. Redhat is primarily a support and consulting organization. If you want to run an SAP system on Redhat, for it to be certified (and hence supported, which anyone buying an SAP contract will want) it has to be on Redhat Enterprise Linux with the support contract that goes along with that. In this case support can easily go all the way up to e.g. patching the kernel / glibc / whatever, and Redhat can charge a premium for offering that kind of in-house expertise.
The MySQL / Qt model is a mixed proprietary / open source model. They offered their libraries under the GPL (instead of the LGPL, BSD or other more permissive licenses) meaning that if you wanted to be able to distribute their libraries as part of a proprietary application, you had to pay for a license.
Since nginx is BSD licensed, I assume they are going for the prior.
Qt is under the LGPL. So long as you do not modify the main libraries you are free to link against it with a proprietary app and can distribute the binaries so long as you also provide source when asked.
Comments
There are two main models -- we could label them the "Redhat" and "MySQL / Qt" models.
The Redhat model establishes a brand, often the original source for the product / project and uses that to market their expertise. Redhat is primarily a support and consulting organization. If you want to run an SAP system on Redhat, for it to be certified (and hence supported, which anyone buying an SAP contract will want) it has to be on Redhat Enterprise Linux with the support contract that goes along with that. In this case support can easily go all the way up to e.g. patching the kernel / glibc / whatever, and Redhat can charge a premium for offering that kind of in-house expertise.
The MySQL / Qt model is a mixed proprietary / open source model. They offered their libraries under the GPL (instead of the LGPL, BSD or other more permissive licenses) meaning that if you wanted to be able to distribute their libraries as part of a proprietary application, you had to pay for a license.
Since nginx is BSD licensed, I assume they are going for the prior.
Qt is under the LGPL. So long as you do not modify the main libraries you are free to link against it with a proprietary app and can distribute the binaries so long as you also provide source when asked.
See: http://qt.nokia.com/products/licensing
Historically, that wasn't always the case: http://en.wikipedia.org/wiki/Qt_(framework)#Licensing
The LGPL option was only added in 2009. Prior to that, various licencing mechanisms were used to extract revenue on various platforms.
(Eg, at one point the *nix version was GPL'ed but the Windows one wasn't, so you needed to pay to ship it on Windows)