More specifically it was where they were not received and the people would not listen to them.
There are few places which are more hostile or unwelcoming to Christian evangelists than Islamic communities.
So once again.... my point is that the Bible DOES NOT teach that it is ALWAYS right to preach the Gospel, everywhere at all times.
It does not mean they didn't try. The passage you quoted says "shake the dust from your feet". Clearly, they were already in the town if they have the dust on their feet. Now, I agree with you that if they are not welcomed and are not listened to, then they should "shake the dust". However, I do not believe the passage implies one should not try.
As far as your position that the Bible does not command that the Gospel be preached at all time in all places, I will say two things.
First, here is a passage that was actually quoted in #41 above.
Jesus said "Go therefore and make disciples of all nations, baptizing them in the name of the Father and the Son and of the Holy Spirit, teaching them to observe all that I have commanded you..." Matthew 28:19
Note, it does not say some nations.
Second, even beyond direct commands, like the Great Commission, there is an overall implied message that the Gospel should be spread anywhere and everywhere. Therefore, I would ask you provide either specific Biblical commands supporting your position or make the case that the implied command is consistent with your position.
In an effort to expediate this debate, I would point out that early in Jesus' ministry, he focused soley on preaching to the Jews. However, after the faith exhibited by the Centurion (among other events) he widened his audience. Any remaining doubt about the completeness of the potential audience can be put aside by Paul's vision on the road to Damascus.