Layer 2 rate limiters can be configured ONLY if the global switching mode of the system is

NOT "truncated" mode.

You can see it using the following command:

6500#sh fabric switching-mode

Global switching mode is Truncated << ============= !!!!!

dCEF mode is not enforced for system to operate

Fabric module is not required for system to operate Modules areallowed to operate in bus mode Truncated mode is allowed, due to presence of DFC module

Module Slot Switching Mode

4 Bus

5 Bus

8 Crossbar

9 dCEF

6500#

I am assuming your 6500 is also in truncated mode(When the switch contains a fabric-enabled module and a nonfabric-enabled module, the fabric-enabled line cards operate in truncated mode) and hence default layer 2 rate limiters have been disabled. In newer IOS, the clarity of the message has been improved and should look something like

%MLS_RATE-4-DISABLING: The Layer2 Rate Limiters have been disabled, as switching is in truncated mode.

There will be also message:

3d19h: %MLS_RATE-4-NOT_SUPPORTED: This functionality is not configurable.

Hope this helps