Because this isn't really a beta server in the usual sense of the word. Instead, it's more like a normal server where features are released earlier, and you get paid diamonds every so often. It's basically a test to make sure the features are stable on a large scale, and to see feedback from players on balancing/fairness, even if that last point isn't always listened to. Additionally, this server is often used to see how players on live servers would react to the features. For example, if you gave everyone a sandbox world, they would just give themselves every resource or tons of event currency, and that doesn't provide an accurate representation of the game in a live setting.
(Most likely) The devs have their own private testing server for the majority of debugging and probably an alpha server that would be what is considered a normal beta test, with more of a sandbox type environment for testing features in a variety of scenarios. We saw a bit of this with I think SAV or SAJM (it's been a while and I'm too lazy to look it up) where some players got access to pre-made cities for the purposes of testing the age.
Overall, the reason you are able to pay for stuff on Beta is because they want you to. They want to see ahead of live who is purchasing what packages, where they can make more money, etc. Why do you think the Forge Bowl coach is more expensive? Why did they switch to paid event passes? Because even though we complain about them, many players are still paying for them simply because they are in the game. The cost to develop and implement these features is much less than the revenue they make, so while it does ruin free to play gameplay, in terms of purely supporting the bottom line, they are the logical decisions.
Time and time again, the only way to express your true thoughts is to vote with your wallet. Don't like the level 9 + premium level 10? Then don't purchase the event pass. The only way we can expect to see change is if you realize that spending money on these features only encourages more of them to be added. If you don't like it, don't buy. That feedback will be listened to much more readily than anything anyone on these forums writes, as it provides data to Inno on what the market of the game will bear rather than just unquantifiable player sentiment.