From the CNN article above:

Israel had offered to deliver the aid to Gaza, but passengers rejected the offer, according to a statement from Michael Martin, Ireland's minister for foreign affairs.
Israel said the offer -- formulated with Ireland -- called for representatives of Ireland and the activists to accompany Israel during those steps.
"Unfortunately, the activists rejected this offer, and consequently, the Israeli Navy boarded the ship with full compliance of the ship's crew members, and redirected it to Ashdod port," Israel said.

I guess the "activists" primary goal is not to get aid to Gaza. Israel tries to do the right thing but they activists reject it. Nice. Goes to show that they will disagree with anything Israel does, even when they play nice.