The Solr 4 spatial fields use AbstractSpatialFieldType and by default only filter and supply 1 as the constant score. For sorting or boosting, you can add the local-param score="distance" (or recipDistance) option to have the score of the query be as specified. However this query still filters, and in some cases this is redundant. For example you probably already have a filter query doing the filter portion, and then you are again using the shape reference here for a boost query.
The change is a simple matter of returning the FunctionQuery and not wrapping it in FilteredQuery.