Isn’t that code in tail recursive style?

Scala can only optimise this if the last call is a call to the method itself.

Well, the last call is not to allStrings, it’s actually to the :: (cons) method.

A way to make this tail recursive is to add an accumulator parameter, for example:

def allStrings(expr: => String, acc: List[String] = Nil): List[String] =
  expr match {
    case null => acc
    case w => allStrings(expr, w :: acc)
  }

To prevent the accumulator leaking into the API, you can define the tail recursive method as a nested method:

def allStrings(expr: => String) = {
  def iter(expr: => String, acc: List[String]): List[String] =
    expr match {
      case null => acc
      case w => iter(expr, w :: acc)
    }
  iter(expr, Nil)
}