I think it is wonderful that Guinness made this decision and they should be applauded for it. The St. Patrick's Day Parade tried to exclude the LGBT community and this shouldn't be allowed. I am so happy to see companies stand up for what is right, especially given their strength within the United States.
I believe Guinness made the right decision in pulling out of the St. Patrick's Day parade due to discrimination. The parade's policy of not allowing sexual orientation to be displayed was considered discriminatory by many. As a sponsor of the parade, Guinness was in a position to lose a lot of business had they not pulled out.
One of the advantages of living in a free capitalist society is that companies are allowed to make statements about societal issues. Just as Chik-fil-A should be allowed to say that they oppose homosexuality (and they had to bear the brunt of the public outcry), Guinness should be able to pull out of the St. Patrick's Day Parade because it is exclusionary. Since this fits best with the company's stance on the issue, Guinness made the right decision.
No, Guinness did not make the right decision to pull out of the St. Patrick's Day Parade due to discrimination, because St. Patrick's Day is probably Guinness' biggest day of the entire year. They are trying too hard not to offend anyone and in doing so they are offending people who just want to have a good time.