単に評価順じゃね？？？

gistfile1.txt

scala> val txt = "foo" + "bar" match { case s => {println(s);s} }
foobar
txt: java.lang.String = foobar

scala> val txt = "foo" + ("bar" match { case s => {println(s);s} })
bar
txt: java.lang.String = foobar

だから

  val filePath = "src/main/webapp/" + path match {
    case tentenRegex() => throw new Exception
    case "/" => "/index.html"
    case _ => path
  }

  val filePath = "src/main/webapp/" + (path match {
    case tentenRegex() => throw new Exception
    case "/" => "/index.html"
    case _ => path
  })

にすればいいのでは？？？