Why "MUST" encoder emit Context Update

http://tools.ietf.org/html/draft-ietf-httpbis-header-compression-08#section-5.1

   After applying an updated value of the HTTP/2 setting
   SETTINGS_HEADER_TABLE_SIZE that changes the maximum size of the
   header table used by the encoder, the encoder MUST signal this change
   via an encoding context update (see Section 7.3).  This encoding
   context update MUST occur at the beginning of the first header block
   following the SETTINGS frame sent to acknowledge the application of
   the updated settings.

Why is it a 'MUST' for the encoder to emit a Context Update opcode?  The SETTINGS ACK (from encoder-side to decoder-side) communicates that the encoder has made the corresponding accounting change and implies the encoder will use the decoder's entire table space.  If the encoder does not want to stretch itself to the full extent of the decoder, only then is it a 'MUST' to emit a Context Update opcode in order to advertise what subset of the table space it will use.