.. well I just installed it on a clean 6.5b2 box and it worked fine, no errors!
I ran rpm -qR against the package and it does not list the devel module as a dependency:
[root@test amavis]# rpm -qR zlib
/sbin/ldconfig
/sbin/ldconfig
rpmlib(PayloadFilesHavePrefix) <= 4.0-1
rpmlib(CompressedFileNames) <= 3.0.4-1
ld-linux.so.2
libc.so.6
libc.so.6(GLIBC_2.0)
libc.so.6(GLIBC_2.1)
libc.so.6(GLIBC_2.1.3)
Have got an older zlib already installed courtesy of another contrib?