Do you also reject free coffee, cakes and random ornaments ?
Yes, you might do if you're afraid someone might poison or bug you, and quite right too.
But in general, we trust people based on their reputation. The conference organiser's reputation will be that they might nag you to sell their conferences, but they probably won't try to drug, bug or infect you with malware. Because they have some integrity, and don't want to be demonised.
Why is the branded USB drive any different ? You probably shouldn't accept it from someone who comes up to you in the street. Same goes for the other things I suggested - if someone in the street offers you cookies you make a judgement based on your experience and prejudices about whether to take them.
The USB stick isn't any different. There is a possibility of accidental or intentional malware. You can choose to trust it or not according to your usual threat model. It doesn't make the marketroids stupid for offering it, nor does it make you stupid for accepting it.