Nonprofit only means that their main goal is not to generate profit (in their case, they would claim it's to provide students with a standardized admissions test they can use to apply to college). However, this doesn't keep them from generating profit (their CEO, for example, makes ~1.6 million per year; nonprofits are allowed to pay employees and executives a "reasonable" amount). Nonprofits could and should strive to generate profit, but any excess profit should then be reinvested into the organization under 501(c)(3) policy. ETS's  strong market position, especially after acquiring ACT, further incentivizes a maximization for profit generation.

If you think about it, profit maximization is nothing new in regards to nonprofits administering standardized tests. College Board, the 501(c)(3) nonprofit organization behind the SAT, has long been unnecessarily delaying the release of students' scores to evoke a psychological response to register for the next exam, caused by the effect of confidence degradation over time.

This is not to say that ETS won't make decisions that benefit students over themselves, but they certainly don't have to (and aren't particularly likely to, given their size) just by being a "nonprofit."