"Breaking ties" was a poor choice of words.
It appears that Systrom and Mike Krieger (the other cofounder) together held a majority stake. Assuming the two of them decided to accept the Facebook deal, then they are indeed acting in the best interests of the majority of shareholders, no?
Why do things get murky when the CEO/board/controlling interest is the same? They started and built the company, and structured their funding deals such that they were able to keep majority control wrt voting rights. If they want to screw over the minority shareholders, I think that's pretty lame, but that should be their prerogative.