There are two different issues here. 1) A bad security vulnerability at GitHub. 2) Poor design in Rails that makes it easy to produce security vulnerabilities.
Igor found 2, and got ignored by the Rails team. His frustration led him to publicly demonstrating 1, which caused a whole lot of people a whole lot of trouble.
The people that are irritated at him are irritated at him because of 1, not 2.
I won't try to impute motives but I think he did it this way because he felt like he was being treated poorly.