On the current German datasheet for the 300 Series Small Business Switches the free software update term seems to be limited to 12 months :-(
"Darüber hinaus bietet Cisco für 12 Monate ab Kaufdatum kostenlose Software-Updates mit Bugfixes"

see http://www.cisco.com/c/dam/en/us/products/collateral/switches/small-business-smart-switches/300_Series_Switches_DS_FINAL_2757.pdf

which is translated to English
"In addition, Cisco offers software application updates for bug fixes for the first 12 months following the date of purchase"

So I can't see that the software upgrades are (always) FREE of charge.

So maybe the translator of the German document made a mistake (only free telephone support is limited to 12 months in the current English version) and you should have the document updated or your statement of free updates is limited to US customers.

But if your statement is correct, the reason for warranty extensions is not clear (for what do they exist and claim to offer major and minor upgrades free of charge during the extension term, when all upgrades are free)?

Maybe you could clarify this?