Think about it from this perspective. When you get married you're legally saying to the court/IRS/government that you and this person and now partners and are sharing life duties and trusting one another. You start doing your taxes together, you're legally binding one another both fiscally and through the courts.

There's a reasonable expectation when this happens that your partner is contributing to the relationship in all aspects. Did she stay home and tend to their domestic life or contribute in other aspects that have unquantifiable levels of worth. Did she contribute knowledge and expertise to him as counsel at home, tend to his mental health, etc. Cook his meals or take care of his pets/home/family while he was tending his duties at Amazon, allowing him to do those tasks. This can go both ways for either partner. Contribute to PR as being a faithful wife and maintaining an image of stability to their board of investors, or to the company, or to morale. The list goes on and on.

The idea is that you're protecting both parties in the legal union by admitting that while married, one spouse may be carrying the weight in other capacities that are then allowing the other partner to be a workaholic, spend long hours at the office, etc etc. Jeff can eschew certain responsibilities because he legally stated to the courts he tied his life to his partner and they'd share in those responsibilities and fiscal needs.

Without needing to prove it unreasonably, there's no way to say that she wasn't integral to Bezos's ability to run Amazon and create the company without her help in his life. That's why she deserves 50%.