I am rather surprised that someone actually approached you about this. I would, if they do so again, remind them that during the liturgy our focus is supposed to be on our prayer, instead of those around us. Also, while many people do practice this, it is far from a requirement. I don't believe either of the priests at my parish cross themselves at every petition, and it seems that most of the people who cross themselves at each petition don't even cross themselves at each petition (evidently, the crops aren't really that important...)
You took the words right out of my mouth. Are these individuals praying, following the service, or watching what everyone else is or isn't doing?