Your own source says why. It is not against the law. The law in turn is trumped by Sharia law. According to the local experts, Sharia law allows marriage at 10 years old. Apparently, according to the judge in the case, even at age 8.

Make up your own mind about what this implies about Islam and Sharia law. If you have been reading the news the last few decades this should not surprise you in the least.

Quran and sharia law allows that!

Even Islam’s prophet, Muhammad, married a six-year-old girl named Aisha! He consummated his marriage with her when she was 9. He was then, 54 years old.

It’s wrong to do such things like marrying very young girl to reconcile a debt. Quran does not give permission for such brutal acts. Islam allows marrying a woman/girl after she attains age of puberty, with her will only.
If Saudi Sharia law allows marriage, it’s their headache. Sharia laws are made by human & are very outdated. Islam allows & gives full liberty to woman, to choose man of her choice. No one can force, otherwise its forfeited marriage. Alcohol is prohibited in almost every religion, but how many really don’t drink? Actually drinking is fault of followers of religion not a fault of religion itself.
Yes, Prophet Muhammad, married a 6year old (no one knows exact age) girl named Aisha on the will of her father nearly 15000 years ago. He consummated his marriage with her when she was 9 or so (no one knows exact age), but not before puberty/sensibly for sure with her will.

First, the Prophet Muhammad married Aisha when she was considered to be a young lady, not 6 or 8. This was more than 1400 years ago when child marriage, aging and life in general was was very different from what is it today. The man in the article married the little girl as a way to reconcile a debt. Both men should be severely punished. There is nothing wrong in Shariah law to change a old, ancient ruling if it is to protect the innocent or kept society whole. Many religious leaders in Saudi and other nearby countries are not well-educated, live in a cloistered society and are "book" knowledgeable. It is a horrible shame. Like many other societies, Muslim and non-Muslim those with power can interpret the law to their benefit although there are others who protest. To be honest, very few Muslim societies embrace the Saudi lifestyle. Saudis are not necessarity the template for Islamic society and in my opinion, many of them are arrogant, ignorant, backward people who continue to bring ridicule to the Muslilm Ummah. Masha Allah.