This is a complex area of law, both regulatory, fiscal and contractual.
Note that technically SB is a type of CFD trade. In the EU and UK, SB is regarded by the financial regulators as CFD trading. In the UK SB is legal and is tax-free, whereas CFD trading is legal but is not tax-free. In many countries CFD trading is permitted, but SB is not legally permitted.
Some SB providers are also CFD providers so they can offer both types of trading service. But for some of their clients they will have to offer a CFD trading account, as in the client's country, SB is not permitted.
My assumption is that whether SB is legal or not and whether it is taxed or not is a decision for the country's regulators based on how likely SB services are to generate significant taxable company profits and how risky SB services would be to the financial services industry in that country. There might also be an argument that incumbent brokers, who do not wish to see competition from SB, influence the regulators to not allow it to be started up. None of this has much to do with individual traders, the primary role of regulators is to ensure stable financial services industries, not protect consumers.