Not a point of contention.
What if the billboards did not mention terrorism but just advertised the mosque and said, eg, say Happy Ramadan?
As religions go, I don't like Islam...but they would have every right to put such a billboard up. I certainly recognize the distinction between Islam and modern Christianity in their treatment of members of other religions, which is why I prefer to live in a country dominated by Christians rather than one controlled by Muslims.
While I plan on traveling the world when I retire in a few years, Saudi Arabia is not on my bucket list!