Sollozzo doesn't expect the Corleone's to know where the meeting is being held, which is why he wasn't that worried about security. Sollozzo presumably didn't buy protection like the Corleones did. Either that or Sonny was willing to take on that family as well. This brings me back to my original point. Once the Corleones knew where the meeting was being held, they should have been able to take out Sollozzo themselves, and didn't need to rely on Michael to do it.