In 2004 I had the same Question, so I e-mailed Carlon. This was what they told me.
" A male Adapter is an adapter that provides a smooth insulated surface to protect the conductor. 300.4 F States that for ungrounded conductors 4AWG and larger the conductors shall be protected by a fitting that is smoothly rounded an is an insulated material, Which Carlon's Male adapters are. This is a gray area in the NEC. Several inspectors in the country still require bushings." This was stated by David Kendall, Director-Industrial Affairs, NEC CMP 3 & 8.
Since I have received that e-mail I no longer require bushings. But I do think that I is a good idea to install them.