At the start of the fight, if a fighter puts a glove out to touch hands and you come over the top like that then I definitely think it's a total cheap shot. Not illegal, but definitely cheap.
However, I remember a few months back when Mayweather KO'd Victor Ortiz and people were crying foul. The difference in that situation was it wasn't the start of the fight, and Ortiz had just headbutted Floyd in an attempt to break his nose (which Ortiz admitted). Immediately after the headbutt, Ortiz was acting apologetic and touched gloves with Floyd. Then when the Ref started the fight again Ortiz put his hand out a second time and Floyd just came straight down the pipe and knocked him out. I defended Floyd then, and I still defend him today. In the end, this is the hurt business not the hugs and kisses business, and if you take a cheap shot at me you better expect to get one coming back. Ask Evander Holyfield how throwing all those headbutts against Tyson worked out for his ear that Tyson decided to bite off.