No, you can't do that. You've removed a verb "is" and expected "lies" to cover for it. It doesn't work. You could do it if the first verb was "is".
Anyhow, you could almost get by with: "The focus of the speech category lies not in the offensiveness of the idea but is on the mode of expression." You'd need to include 'is'.
Why are people so averse to attributing apparent textual errors to laziness, incompetence or poor English skills? I'm sure they're the main causes.
In this particular case, I would say because the person who wrote the sentence is a law professor and is thus likely to have a better command of the English language than I do, but also because I'm assuming that his book was proofread by editors. I may, of course, be wrong on both counts.