think that she did 'sign' an informal contract with the British public.
Don't be ridiculous. Marriage is marriage. Absolutely no man or woman enters into a life of obligation to a people group as a result of choosing to spend their life with another individual. Apart from anything else, this idea is ludicrous because the British public is unable to decide what, if anything, it wants from Kate (or any other royal) in the first place. Any such 'contract' would be subject to innumerable drafts and would, in all probability, be shredded by both the feminist board on mumsnet and any employment authority in the country. So let's not lampoon someone for choosing to spend their children's precious baby years with them instead. After all, no one thinks she's accomplishing anything by handing out shamrocks anyway, right?
If anything, she entered into a contract with William to live his version of a royal life - she agreed to do her best to create a happy home, which is at the heart of any successful family and has a great deal to do with what the members of that family go on to accomplish. I suspect we are judging her in error when the decision-maker is probably him. The pity is that his decisions are in all likelihood driven by love for his children and a knowledge that good parenting is the best way to create healthy individuals - the kind of individuals we would want in a royal family, if we want anyone at all.
And therein lies the problem. The monarchy is no longer wanted, on the whole. To be anti-monarchist majority, anyone marrying into it looks like they're defecting. Also, Kate's a woman and mumsnet loves to knock women, especially when they're privileged, stay at home to raise their children, and aren't known for having an irreverent, acerbic wit. She was a sitting duck for contemptuous Intelligent British Woman - and doubtless knew it.