std.algorithm.splitter on a string not always bidirectional

[Steven Schveighoffer via Digitalmars-d-learn]

auto sp1 = "a|b|c".splitter('|');

writeln(sp1.back); // ok

auto sp2 = "a.b|c".splitter!(v => !isAlphaNum(v));

writeln(sp2.back); // error, not bidirectional

Why? is it an oversight, or is there a good reason for it?

-Steve