It can be reproduced in rhel6.5 host.
host info:
# uname -r  && rpm -q qemu-kvm-rhev
2.6.32-425.el6.x86_64
qemu-kvm-rhev-0.12.1.2-2.415.el6.x86_64

e.g1:...-drive file=/home/my-data-disk.qcow2,if=none,id=drive-data-disk,format=qcow2,cache=none,aio=native,werror=stop,rerror=stop,readonly=on,snapshot=on -device scsi-hd,drive=drive-data-disk,bus=scsi0.0,channel=0,scsi-id=0,lun=1,id=data-disk
(qemu) info block
drive-data-disk: removable=0 io-status=ok file=/tmp/vl.7d7Gen backing_file=/home/my-data-disk.qcow2 ro=0 drv=qcow2 encrypted=0 bps=0 bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0

e.g2:...-drive file=/home/my-data-disk.qcow2,if=none,id=drive-data-disk,format=qcow2,cache=none,aio=native,werror=stop,rerror=stop,readonly=on -device scsi-hd,drive=drive-data-disk,bus=scsi0.0,channel=0,scsi-id=0,lun=1,id=data-disk
(qemu) info block
drive-data-disk: removable=0 io-status=ok file=/home/my-data-disk.qcow2 ro=1 drv=qcow2 encrypted=0 bps=0 bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0
(qemu) snapshot_blkdev drive-data-disk /home/sanpshot
Formatting '/home/sanpshot', fmt=qcow2 size=5368709120 backing_file='/home/my-data-disk.qcow2' backing_fmt='qcow2' encryption=off cluster_size=65536 
(qemu) info block
drive-data-disk: removable=0 io-status=ok file=/home/sanpshot backing_file=/home/my-data-disk.qcow2 ro=1 drv=qcow2 encrypted=0 bps=0 bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0

Best Regards,
sluo

snapshot=on and live snapshots behave different.  Since the readonly=on applies to the drive, a live snapshot should still honor that setting, and not change the disk from read-only to read/write.  Running snapshot=on creates a temporary throw-away snapshot.