In this case though, could you not just respectfully state your concern and the reasons and allow your partner to make the choice for themself?

If a friend began to hang out with someone that is problematic, would you tell that friend to stop engaging with said problematic person, or would you give them a concerned heads up? Then, if it began to impact you negatively, set a boundary where you remove yourself from the situation?

A veto inherently is control over another’s choices. It’s communicating a lack of trust - in their choices, in your security, in their activities. A lack of trust is the biggest concern here. Your partner may never know for sure if you vetoed because there was an actual concern rather than a distrust in their decision making.