Is that legal? You can claim ownership of derivatives??

Sure, you can claim anything.

IANAL, but I don't see why this would not hold up. It is their code, its in their license agreement (which is a contract you agree to). If you do not wish to give them the rights to your modifications, the license forbids you from using it

Sure, but I was of the impression that they can only control rights to use their software, not claim your own - I'm not sure if "which is a contract you agree to" is true if nothing is signed.

Note: by reading this notice you agree that all that is yours is mine, and all that is mine is mine also. ;-)

Your note does not work because I am not agreeing to anything. You either are agreeing to the license implicitly by using the product, or you are not allowed to use it (and would therefore use code which you dont have a license to)

If you read the message, you agreed to the terms. If you didn't read it, then how can you comment on what it says? :)

if you havent read it then you are using software which you may or may not have a license to, and you're just hoping that you are allowed to use it.

The license applies, wether you read it or not. The only difference is if you are aware that you are breaking it, or not.

> If you do not wish to give them the rights to your modifications, the license forbids you from using it

Unless your use qualifies as fair use

In my observations, "fair use" is often misunderstood. People violate copyrights thinking theirs is a fair use when it's not.

Is "fair use" on software even a thing?

(I know some jurisdictions have basic exceptions like "if you own software you can do what's needed to run it", but modifications?)

Well it's an option - either you follow the terms of the license agreement and give up ownership, or you're in violation of their copyright. (Or you contact them and work out better terms.) They can put any terms they want in the license agreement, if you don't agree to them you don't get a license.

Not sure if this conditional violation stuff would hold up..

Think of it with money instead. If they charge $20 for a license, then you have the option to pay, or to be in violation of copyright. Here they're requiring a transfer of IP instead of money but it's the same deal.

Not really. It's not the same as paying. Once you pay you'd get confirmation.