I believe that formhelper generates its fields with use of the schema and since the items in a hstore can vary, it can't generate those fields automatically.
Apart from the implementation details, you can just iterate over the values in your object (source):
<% f.grid.attributes.try(:each) do |key, value| %>
<%= f.text_field key, :input_html => {:value => value } %>
<% end %>