It likely has to do with how the TCAM is programmed (see edit below - Brocade refers to it as CAM). I'd definitely be interested to hear why adding a completely separate ACL (vs just an entry in an ACL) would require you to "reapply the ACLs". Do you have a source for that quote? If you don't have an SE assigned to your account, I do have contacts that I can ask and get clarification on this.
While not Brocade, I have had experience with vendor hardware where modifying an ACL "in place" would cause certain rules "further up" in the ACL to be completely ignored/stop functioning. Until the bug was actually fixed, the only workaround was to delete the ACL itself, and then recreate it with all of the rules before applying it to any interface.
Edit: Turns out my original thought was correct - from NetIron documentation:
How the Brocade device processes ACLs
The Brocade device processes traffic that ACLs filter in hardware. The Brocade device creates an entry for each ACL in the Content Addressable Memory (CAM) at startup or when the ACL is created. The Brocade device uses these CAM entries to permit or deny packets in the hardware, without sending the packets to the CPU for processing.
And then further down on the next page: