Last night, I was walking back to my car when I saw something horrible. Right in front of me, I saw about a half-dozen young men attack a homeless man.

I didn't think much was happening at first: just some kids yelling, maybe they were drunk, maybe just excited. But then I saw a taller man, in ratty pants and a poncho with a scruffy beard, get hurled against the entrance to the Portland Outdoor Store on the corner of SW Third Avenue and SW Oak Street. He endured several punches to the head and upper torso before the group took off.

I immediately called 911, and the operator told me police were on their way. They were: as soon as I hung up the phone, I saw the flashing police lights speeding toward the intersection.

I watched as officers questioned Thomas Lundahl, the homeless man. The men attacked him after he saw them try to steal a woman's iPod and he told them to stop, Lundahl said. Lundahl told the officer he was a little banged up, but all right. When asked, he said he did not wish to press charges, even when told that another officer had stopped a group of men that may have been involved in the attack.

The whole thing left me a little rattled, and I couldn't help but wonder: when is it up to the police to arrest someone, and when is up to the victim? This is especially concerning when that victim is homeless, has no money for a lawyer and limited resources to get through the day, let alone navigate a successful prosecution.

Wednesday night’s attack was a low-level assault, says Mary Wheat, a public information officer with the Portland Police Bureau. “The victim has to want to press charges,” Wheat says of this case. Without a victim, “the district attorney can’t take the case because there’s no one to represent.”

That’s not how it works for all assaults. Charges are brought in domestic violence cases regardless of whether or not the victim agrees. The same goes for assaults on children. Shootings would be handled differently, too, Wheat says.

The homeless are particularly vulnerable to low-level assaults like the one I witnessed. As the Mercury reported in February 2008, more than 40 percent of Portland's homeless have sustained concussions from being attacked while sleeping. That the attack on Lundahl happened when he was awake makes it all the more brazen and frightening.

It's scary that we're willing to let people keep walking the streets when the most vulnerable of victims decide not to press charges. How does that help the rest of us?