I wonder if this might not be something else. When you study a language in a classroom, you frequently engage in conversations on various topics, whose sole purpose is to make you practice much more than to actually have a well-thought conversation. As such (and here I speak from my own experience only), it is much more common to defend positions in which you don't actually believe, for the sake of argument or maybe…
Neither of the two languages a bilingual person speaks is considered foreign. Otherwise the person would not be bilingual. What would be the result of such a study? Keep in mind that in linguistics the difference between being a native speaker and having learned a language after the critical period is considered huge.
Being able to communicate in two languages is not the same as being bilingual. The study in question suggests that thinking/communicating in a language that one is less than fluent in leads to more utilitarian decisions.
The important thing here is fluency, not nativeness. I am fluent in German but I started learning it at 12. German is not a native language for me but I am fluent in it. As such the study suggests I should be equally utilitarian in it and English, my mother tongue. But my French and Mandarin are both quite bad so I should be more utilitarian in them because I must be more deliberate.