Yes, but customizing and patching is not that uncommon for commercial software (Qt is used a lot in the embedded space, for example, where patching and shaving away is common).

Also, businesses do pay for support as insurance; say tomorrow Windows patches something that screws up a bit of Qt your product depends on, what are you going to do? Waiting for a reply on a random mailing list and hoping someone has time to fix it, is not going to cut it.