Discuss this with your manager, or with some other person in a suitable decision-making position. If you and your colleagues are doubting the value of the material that you already took, someone in your organization should know this. They may want to re-evaluate this course for their company.
One question that I'd recommend you consider is: Is there any indication that you and others have been singled out for this training? Or is is something that all employees are streamed to? If people are being "selected" for it, then the follow-on question is, is this is a good way, as in being groomed for promotion, or in a not-so-good way, as in, they think you may need remedial social skills. If either of these is the case, then opting out may have very specific risks.
It's a red flag for me that the travel itself is unpaid time. Is this on a weekday (regular work day) or a weekend? Especially if it's a regular work day that you're being docked because it's spent in travel to a mandated work event, I think you've got a valid complaint.
