(TP-link started locking down at least some models, in response to FCC rules, and some manufacturers don't allow you to downgrade, but there is no legal requirement to do so and often ways around it. Other manufacturers like Linksys openly advertise their routers for use with OpenWRT)
http://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELE...
http://www.tp-link.de/download/Archer-C7_V2.html#Firmware 1.For Archer C7(EU)V2. 2.The EU firmware was specialized for CE certification and can't be downgraded to other version, please click here for choosing your region and selecting the most suitable firmware version to upgrade. 3.Old firmware's configuration file can be imported into this new firmware; 4.Your device's configuration will be lost after upgrading, which means you need to configure your device again
All routers are doing the same. EU law.
Citation still needed.
a) I reviewed your eur-lex.europa.eu link, and didn't find anything that prohibits installing other firmware. Please cite the chapter/article that's relevant.
b) re: TP-Link firmware. It is true that TP-Link has added a technical means that makes it impossible to install certain other TP-Link firmware (the "downgrade" they mention). However, there are other firmware distributions (LEDE/OpenWrt/DD-WRT/others) that have the capability to install over the TP-Link factory firmware.
And last I've heard about it they extended the validity of the old rules for undefined time (due to most of the standards not being ready), but I can't find a good source for it now, so I might have gotten that wrong. My mistake for not checking better.