If you "import" (read: steal) text from a source which is not compatible with the CC-BY-SA 3.0 license, then you may have actually violated copyright. At the very least, you've given someone else's text a legal status they didn't originally anticipate. This is the most serious form of plagiarism, and may force an admin to block you. Taking from CC-BY-SA-compliant sites is still less desirable than simply writing articles from the ground up, but it may be allowable in certain circumstances.