Beyond any historical argument that explains why the beta is paid, can anyone make an argument that doesn't simply explains it, but one that justifies it? Even more, on the argument in favor of paid beta under the premise that testers who pay actually do some testing and reports; it is not true that selling beta ensure proper testing, and it is obviously not the case that charging money for beta testing is the best method to filter testers. It is simply wrong. But, on the other hand, making open beta is likely the wrong way to go too, as it will spoil the release by killing the hype and might saturate the servers as someone argued once, but come on, you can still make a closed free beta.
There is an argument going around here stating that open beta will get filled with casual players and beginners. I totally agree, but from that idea, it takes only one logical step to come up to the idea of a closed and selective free beta to prevent that situation from happening. FD could choose old players registered before a certain date, and with a certain amount of in-game hours, and then spam them with something more noble than charging for money of something that should be rather a paid or volunteer job (not a privilege), and a totally necessary part of their own job, and something that have proven to fail consistently in the history or their own game.
FD should stop advertising beta testing as something exciting to do because it is not. If excitement is the motive, then it is Early Access (the Early Access of the Early Access), not beta. And they should also be honest while advertising it, as when they claimed that 2.3 beta was arriving in its full swing. And about the argument to support FD on the development of ED, if that is acceptable, then it also is to send them a 6 euros donation instead and go wash their dirty dishes, all the while you receive invitations to do so on your email.