Arbitration is overwhelmingly resolved in favor of corporations. The company pays the arbitrator, which means they will generally rule in their favor if they want to continue to be hired. Complainants get a fraction of the amount of money they'd get from a court case from arbitration, and it keeps the public from knowing what the company did. That's why so many companies are trying to force arbitration clauses on consumers.

It's speculated that the reason why Steam backed down from their clause in this case is that it was getting too expensive for them. Paying so many individual arbitrators and lawyers was costing them way more than resolving a single class action lawsuit. Hopefully more companies are forced to come to this realization in the future.

Edit: Article about why they may have removed the clause TL;DR Valve doesn't want to deal with 50,000 separate court cases at one time