Manufacturers and vendors of these active devices lie about their legality. The FCC has issued written clarifications and replied in writing to inquiries. In both instances they make it clear that it is only legal for licensees to install these BDAs. There are no licenses issued just to operate a BDA.
One problem is that many of these devices, especially those sold to repeat Nextel signals, also repeat the signals of many other licensed systems.
Much accurate information on this subject is available here:
Rik Rasmussen Two Way Radio Directory