From the GM Engineering Center bulletin http://service.gm.com/techlineinfo/radio.html
"NOTE: It is recommended that a fuse be placed in the transceiver negative lead to prevent possible transceiver damage in the event the battery to engine-block ground lead is inadvertently disconnected."
The failure mode is one where a high (relatively) resistance or open occurs in the battery to engine block cable causing the negative return cable directly to the battery (and the antenna coax shield conductors to become alternate high current return paths to the battery for the starter motor current (until the fuse at the battery in the negative line opens, or the negative wire melts and breaks.) Note that the positive wire in this case does not see any excess current.
Compare this to the different fault scenario where a positive wire to chassis fault occurs and short circuit current flows from the battery thru the frame to the frame/battery ground cable (until the fuse at the battery in the positive line opens or the positive wire melts and breaks the circuit.) In this case, unless the battery to chassis ground cable also fails w/ high resistance or opens, the negative fuse doesn't see any excess current.
Connection to a point slightly removed from the battery (e.g. engine block, frame ground point or SEO connection (if you have one) may eliminate the need for the Neg fuse, but may increase noise on the power lines at high current draw.
"...and connect the negative lead to the battery body connection point (identified by a short #10 AWG or larger wire running from the negative battery terminal to the body of the vehicle).
4.) Connect the positive and negative leads to the Special Equipment Option (SEO) wiring provided for this purpose."
Note that is is not the primary function of either of these fuses to protect the radio from internal faults (though they may as a secondary function)), but to to protect the wiring leaving the battery. The radio primarily is protected by the fuses in its leads at the radio end.
As well, while the wire failures are glibly described above as "melts and breaks..." the reality is there is likely an accompanying wire insulation and perhaps nearby objects fire, never a good thing in the engine campartment or under the dashboard, carpeting, etc. of a motor vehicle!
Ford also has (briefer) recommendations for land mobile radio installation/wiring, see: http://www.fordemc.com/docs/download/Mobile_Radio_Guide.pdf
Hopefully the links are of assistance.