Free software licenses are social things, not just legal ones
In theory software licenses are (pseudo-)legal instruments and can be considered purely on that basis, doing your best to figure out what would be found legal or improper under one. In practice, of course, this is nonsense. Every common free software license has accreted a social community around itself (or sometimes several), with its own philosophies, norms, views of the world, and priorities on what they consider important. As this happens, the actual written out software licenses themselves slowly become transmuted into being only the imperfect realization of what these social communities feel is the spirit of their licenses.
(This focus on the spirit instead of the legalities is part of what leads licenses to be revised over time and to evolve. When the spirit and the legal technicalities drift too far apart, people set out to fix the legal technicalities. Sometimes this leads a community to discover that it is actually several communities with somewhat different views of what their license's spirit is.)
These communities form relationships to each other, of course. Some are hostile; some are begrudgingly accepting; some of them are fully approving. These relationships are generally based on how the communities feel about the spirits of the licenses involved. These community views and relationships matter more than the legal technicalities.
If you find a surprising way to (perhaps) legally combine two things that a community feels doesn't fit within the spirit of their license, the community does not shrug regretfully at their legal error (or your cleverness) and move on. Instead people object, often loudly, and the community becomes angry and hostile to you. In reality people are not picking some legal wording when they chose a license to care about; they are signing up to certain philosophies and they object when they feel those philosophies are being violated, regardless of whether the legal bits let you get away with it.
And a hostile community generally matters. A hostile community is an uncooperative and obstructionist one; it is people who are willing to go out of their way to not work with you and to make life inconvenient for you, even if this has potential collateral damage and hurts their own interests and work. They may not be able to do anything to stop you legally, but they can certainly make their displeasure felt in plenty of other ways.
(Similarly, a cooperative community is potentially a great help in various ways, including being receptive to various kludges to make things look good enough on the legal level if there are potential problems there.)