How exactly ppl get confused with this?
How do you have access to Lich King heroic encounter? By defeating Putricide, Blood Queen and Sindragosa in heroic mode of the same size (10 or 25).
That answers why they have a restriction for heroic version. On cataclysm, heroic raids can have extra bosses and events (one of the new raids will have extra lore/content if you beat it in heroic mode). So, doesn't make sense to let ppl skip hard (and necessary) bosses to get directly to the extra "heroic only" boss, event, etc.
That also answers the achievement questions. Want Bane of the Fallen King? Deal with it in 10man heroic. Want Light of Dawn? Deal with it in 25man heroic as it should be (for now, while in Cataclsym, it will be one achievement for both).
To be honest, the real deal now (since WotLK hardmodes were created) is the heroic mode, Blizz couldn't care less for normal mode. That's why they're kinda of protecting it.
And no, they're not forcing you to raid only heroic ICC if you do one heroic boss. They're forcing you to only raid with that raid lock if you do one heroic boss, you can't do a raid hopping to get heroic kills here and there. You won't be able to do something like "deal with Putricide and Blood Queen 25 heroic with my group, then I'll join that other raid for Sindragosa 25 heroic since my group sucks and then another one for LK 25 Heroic since the former raid sucked at him." You can bring non-saved ppl to your 25 heroic raid, but you can't get saved and join another heroic group that is ahead of your former group on their 25 heroic raid lock.
And forget about DBW in Saurfang 10 ppl, don't confuse flexible raid system with new loot tables for cataclysm.
The only question I have is about not getting saved... if players join a raid to help kill a specific boss... and leave the raid (and probably hearth) with, let's say, 2%, 1% boss health, do they get saved or they just helped the raid to kill the boss? LoL.