In The Godfather during the Sollozzo negotiation dinner, Michael wanted some place with people so that he could feel safe, but later he himself shoots and kills both Sollozzo and McCluskey. So why didn't Sollozzo put any counter measures for such a "just in-case" scenario ? Did he underestimate Michael's capability? I mean, he could have easily put his men as undercover customers ready to shoot Michael upon seeing any threat.
Solozzo indeed puts in place many countermeasures. First and most important, he was with Police Captain Mark McCluskey, who was on his payroll. Those days no one dared to do anything to a policeman, especially a Captain. It means bringing wrath of force and media on the whole family.
Second, Micheal was checked for arms/guns or weapons while in the car sent by Solozzo. So, he was not supposed to have gun. The only man with a gun was the Captain, who was on Solozzo's side.
Thirdly, Micheal was not even supposed to know where they were meeting until the end. Solozzo was confident that he could not plant a weapon or a man at the meeting place. The Corleone family got this information by paying some man from the Solozzo gang.
Fourth, Micheal did not seem that harmful, as he recently joined family that to be until Don was in hospital. In fact, he was not involved in family business that time.
All the above conclude that the only man who was at risk was Micheal. The countermeasures taken by Micheal for having a place with people, it works both ways. Micheal was also not supposed to be able to kill in front of people.