Such owner of the betshop will be sanctioned or their license ceased by the gambling company if found wanting of the very act, so they don't endanger the reputation of the gambling company.
That's not how these gambling platforms work. First of all, there have been blatant cases of theft by online casinos with demonstrable evidence of theft which have gone unadjudicated because there isn't a way to hold these casinos liable. Their gaming licenses are in foreign countries and players don't live in a country with jurisdiction over these companies.
Even if they did, underage gambling/gambling addiction isn't the responsibility of the provider. I'm not sure why it would be. You don't blame the business because the consumer cannot act responsibly. Whether the consumer is underage or not shouldn't incur liability on the provider.