Annoying? Yes. Disrespectful to loyal customers? Yes. Illegal? No. You should read their terms and conditions. They're VERY favorable to GREE. https://dragonsoulgame.com/terms_of_service.html
First, they claim that, by playing the game, you agree to terms of service which aren't ever mentioned in the game. That part is likely illegal, or at least unenforceable in court. But there's more!
Every screenshot of DragonSoul on the internet (including in these forums!) is against the TOS, unless it's on Facebook or Twitter, and then only if it's private.
Anything you post in chat or on these forums can be modified and/or sold by GREE. You continue to own the copyright, but you've basically given them all rights to it. This is one of the reasons I write guides on my own website instead of on game forums.
Here's where it says they can take away your in-game diamonds whenever they feel like it, whether you paid for them or not.
If they don't like you, they can ban you and take away everything you've bought with real money.
Probably my favorite part. They can change the TOS at any time without telling you. So if they replace the whole TOS with "every player must pay us $1,000,000 by March 30th," that's fine. Of course, like the rest of this nonsense, you can't enforce a TOS that no one has actually agreed to. It's hard enough to enforce one that people have agreed to.
They can shut it down any time without notice or liability. I guess they were doing us a favor by telling us.
So... uh... the fact that they want to enforce a TOS that no one agreed to can't be held against them because... uh... you implicitly agreed to it?