By blackmail, you mean they take reasonable precautions to ensure people don't steal from them.
A precaution is an action that is taken in advance to head off an impending problem.
What Steam is doing here is more of a deterrent, in that they are threatening a future action (the account suspension) that will happen after the event (the chargeback), and as a response to it. Unfortunately the threat of that action deters all use of chargebacks not just those that are wrongful; it effectively deters customers with valid complaints from exercising their statutory rights.
A reasonable and effective deterrent should deter the illegal/harmful activity without affecting those that are honest and are themselves being treated unfairly.
Steam's current policy may be effective (by some measures) but it is far from reasonable; it also appears to be of questionable legality but until challenged there is no way to know.
Just because something is written in a contract/license doesn't mean it is (or indeed should be) legal/enforceable, i.e. it doesn't matter if you agree to give up your rights voluntarily, some rights you simply cannot give up. Unfortunately (in many countries) the actual facts of a legal case are of less importance than the depth of the protagonists pockets, and corporations (including Steam) use this fact to effectively bully consumers into submission.
In the US organizations like the
EFF will support consumers on issues related to digital products but it is an uphill battle. They are active in many areas including intellectual property laws and subjects that have been debated here extensively recently such as those related to the "right of first sale" and unfair licensing agreements.
For example,
here is an interesting case being supported by the EFF around the whole ownership vs. license argument for video games. (The link is from last year, but the 9th circuit are due to rule on this next week.)
