Is this some sort of half assed attempt at reverse psychology? In the rain, nobody want's to hang around outside, much. The goal is to get sheltered; usually just scurrying from one building to another as quickly as possible. If you really don't like busterers, then why not let them stay out in the rain?
On the other hand, if the buskerers need someplace to go that's warm and dry, and where there are plenty of people to serenade, come on up to Portland State University, where, "The Campus is Your City"!
Look at it this way, the people who are performing are taking up space that would otherwise be occupied by people who are even less desirable- like the pool of tapeworm feces who was recently arrested for beating the 70 year old employee of the Portland Outdoor Store into unconsciousness with a skateboard.
On the other hand, if the buskerers need someplace to go that's warm and dry, and where there are plenty of people to serenade, come on up to Portland State University, where, "The Campus is Your City"!