You're forgetting that the "Berlin Wall" was actually two walls separated by a strip of "no-man's land" which was mined and patrolled by armed guards and dogs.

You're forgetting that the "Berlin Wall" was actually two walls separated by a strip of "no-man's land" which was mined and patrolled by armed guards and dogs. Anyone who managed to breach one of the walls would not get far. While the guards tolerated souvenir hunters, you would definitely not want to make a break for the other side. Although crossing may have been technically legal, it was still under the jurisdiction of the border authorities; just as it's legal to travel from Mexico to the US, but you can't just waltz across anywhere you want. Remember, East and West Germany were still two different countries, each with their own laws about customs and immigration. The majority of the "wallpeckers" just wanted a souvenir and had no intention (or means) to actually break through steel-reinforced concrete using a hand held pick-axe and hammer. In fact, it took a concentrated effort to breach the wall sufficiently to create an opening you could actually pass through. The initial attempts at grabbing souvenirs (followed by actually removing large sections of the wall with construction equipment) happened at heavily populated areas like Checkpoint Charlie, where the large numbers of people made it harder for the authorities to object to. Full disclosure: I was there and used a friend's hammer and pick-axe to break off a few pieces of the wall.