Why doesn't the extension work in the admin panel for tier prices?
The output for these prices is working in a different way — it doesn't use formatting.
The template app\design\adminhtml\default\default\template\catalog\product\edit\price\tier.phtml contains hardcoded formatting
with 2 decimals: sprintf('%.2f', $_item['price']).
Replacing a major template just for a minor parameter isn't sensible. More importantly, this template may differ from one Magento version to another or in non-standard themes for the admin panel.
So it is easier to just let you edit this template yourself, if needed.