I think, traditionally, the '+' represents a space. I'm not sure what standard dictates this, but it seems to be th

I bet you'll find that the URI spec dictates that it must be escaped when it doesn't have a special meaning to the URI, as is the case here.

One such special meaning was the seperation of keywords in a ISINDEX query (script.cgi?these+are+keyworkds). The query would have had whitespace at those spots, so it was common to replace +s with spaces.