Redistributing BSD licensed code under the GPL could create a PR problem, but it's legally allowable: the BSD license allows redistribution and is compatible with the GPL.
The real problem with open sourcing a previously proprietary codebase is the potential inclusion of third-party proprietary code. If Netscape included some module from Widgets, Inc, under a license that allowed distribution in compiled form only, Redhat would violating the license if they released that code as part of the NES codebase. So they have to comb through the code and the legal archives looking for things like that, and if they find anything, replace it with code they do have the right to redistribute.
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