-1 does not work for HADR, so not used very much anymore. I assume that the reason for 254 limit is the size of the field that stores that value. As to why they want that limit, IBM probably thought that if you need more than 255 active logs, there is something wrong with your application, especially since (as you pointed out) you can simply increase the size of each log file. When I say "something wrong" I mean failure to do periodic commits or close transactions.
M. A. Feldman
IBM Certified DBA on DB2 for Linux, UNIX, and Windows
IBM Certified DBA on DB2 for z/OS and OS/390