Principle of proportionality comes into it (or is that just EU law?).
You're all right, theft is theft but there is the argument that if a firm pursues heavy-handed policies, it is not exactly the best way to promote staff co-operation generally. That firm's management can kiss goodbye any form of substantive staff-led cost-saving (like turning lights off, printing on both sides of the paper, little things that properly motivated staff would think to do of their own volition, which save a damn sight more than a couple of oreos).
Then there's the cost of the action itself. There are administrative costs associated with processing a worker's dismissal. Then the firm will still need someone to do the sacked person's job, so the firm will have to carry the cost of recruiting the replacement (unless they don't replace, in which case he might have an action for unfair dismissal as it was effectively a redundancy so he'd probably qualify for more lolly as a payout).
Shareholders might be within their rights to ask about the origin of this policy at the AGM as it would appear from the above that it is costing the company money. If that's the case, then if it can be traced back to a director, then maybe that director's position may be untenable as they appear not to be fulfilling their statutory duty to look after the company's affairs to the best interests of its shareholders; in fact they are actively working against shareholder's interests by pursuing a policy under which it can reasonably foreseen that there will be a net cost to the company.
And if they couldn't see that there would be a net cost to the company, then perhaps there is an argument that that director has either been negligent or is simply unfit to be a director.
I am not condoning theft here; merely asking, what is the purpose behind the blanket policy? I agree there should be some sanction against the staff member. The idea of a threshold for disciplinary action was mentioned earlier; that threshold should be, the cost to the firm. Knicking a biscuit? Loss of some privilege and a mark on the person's record, to stand against them in the event of future disciplinary proceedings. Knocking off (say) a bottle of First bubbly? Greater loss of privileges and docked pay to recover. Taking (say) cash or MCOs or some item of hardware? Out the door, possible police report. Staff would understand this and, most likely, co-operate. The relevant union would not be able to argue against it.