As I said; the proof is in the pudding. If Blizzard were worried about people charging back 3 years of subscriptions on their credit cards as you say, there is zero chance they would implement something like this. They aren't. Ergo: You think you know more than the people in charge of a multi-billion dollar company and their desire to keep their money.
What you say is utterly irrelevant if it is not born out in reality; the current reality simply does not reflect what you say in any location. If it did, everyone who quits WoW would just charge back the 3 years for free money. I live in Australia, whose consumer protection laws are as strong or stronger than Europe. It still doesn't and won't stop Blizzard banning people if they think it will benefit their bottom line to do so.
You claim this is all the case because nobody has challenged it, but that is utterly your own opinion; as you say, it has never been tested. As a result, the entirety of your opinion is based on your rationalizations of the law and not the applied reality of the law.
- - - Updated - - -
And as a result your steam account was banned in the process, along with any other products associated with it. If it wasn't, you are lying. Charging back to steam, or to Blizzard, results in an immediate and irreversible ban on the account until "the dispute has been settled", which means you ain't getting your account back unless you pay back the chargeback. This is true of almost any large company, since it is the only protection they have against fraudulent chargebacks. You are within your rights to request a charge back, but that doesn't mean the company are just going to say "oh well, sucks for us". Hope you didn't link any other games to steam or play any other Blizzard games you wanted to keep playing.
The TL
R; is that you can absolutely get your money back by issuing a charge back, but it will absolutely mean that you lose your account in its entirety, permanently.