Most of drivers are LPD drivers with filters.
For each of these printers, a script called cupswrapper write down a PPD and a filter to talk to the lpd driver. The cupswrappers, the PPDs and the filters are covered by the GPLv2 licence, but the lpd driver is closed source. Filters are written with Bourne Shell or C Shell. For more information, see the Generated PPDs and Filters documentation page.
The LPD binaries have hardcoded paths (like /opt/brother or /usr/local/Brother), so its impossible to package them following the Debian Guidelines.