This is a deliberate omission - we do not support basic federation yet, and the BINDINGS keyword is really part of the federation extensions for SPARQL (although I just noticed that in the latest working draft they seem to have taken it out of the federation context somewhat).
Certainly seems that it has useful functionality without being used for federation.
parsing of bindings clauses now added to the parser, not yet properly evaluated though.
Implemented evaluation of the bindings clause using a new algebra expression: BindingSetAssignment. Needs some further testing (and possibly a code review by someone other than me) before I consider this resolved however.