Why is privacy scrambling a standalone block in a communications transmitter?
There are two possible scramblers, one meant for security and 
privacy of the information, and the other for making sure that 
the sequence of symbols transmitted over the channel has 
properties that are desirable for digital transmission purposes.
Either or both might be present in a system, or both could be 
absent too. Furthermore, the scramblers might well be 
non-interchangeable since the scrambling needed for privacy 
need not be the same as that required for transmission purposes.

The question then is, where should these scramblers be placed
in the transmitter.  The channel scrambler obviously should be 
between the error-correction encoder (whose output is to be
transmitted) and the modulator that creates the actual signals 
that are broadcast over an antenna or transmitted down a cable
or twisted pair of wires (or recorded on a magnetic medium or
CD or DVD or what have you).  The question of where the
privacy scrambler should be placed is more complicated.
@JasonR has contended in comments that putting it before 
the data compression unit makes the input to the compressor
less compressible.  This is certainly something to think
about, but I am not completely sure about that it is always
a major concern
because the scrambler for security and privacy might be doing
other things than breaking up long sequences of repeated
symbols.  In any case, the location of the privacy scrambler
before the data compression unit may also be dictated by
privacy and security concerns that may have nothing to do with
efficient communication.  If I were in charge of privacy and
security, I would make darn sure that the chain looked like

data --> ENCRYPTION -- > scrambling -->

in my domain before the dsp and comm people even get to see
the stream of symbols.  Let them worry about
how to transmit the stream I give them as efficiently as
possible; my job is security and so I make sure that things are
nailed down tight when they leave my shop!