From what I gather, a lot of the Kirpans in the US are either too small to be a weapon, or have been de-weaponized (fixed in sheath and dulled) to comply with local laws. I don't know what he was carrying or if he was carrying. My very limited understanding of the religion is that only intiated (10%-15% of the Sikhs) are required to carry the 5 articles of faith.

I did pull the Oak Creek municipal code and there is a prohibition on carry concealed fixed blades over 3", but no mention of open carry.

http://www.oakcreekwi.org/documents/municipal-code/Chapter%2011%20Offenses%20and%20Nuisances.pdf