The treatment follows from spread bets being wagers rather than disposals of an asset. That also means there is nothing to offset: a losing year gives no deductible loss, unlike a contract for difference position, which sits inside uk-capital-gains-tax and can generate usable losses.
The exemption is not unconditional. If spread betting is genuinely carried on as a trade or is bound up with other taxable activity, HMRC can take a different view, and the treatment applies to the individual rather than to a company.
Spread betting is offered to UK residents and is not available in many other jurisdictions, including the United States. Where you are tax resident decides the answer, not where the broker is.
This is general information about the United Kingdom, not tax advice. Rules change and depend on your circumstances; check with a professional or HMRC.
Related: uk-capital-gains-tax, hmrc, fca, uk-stamp-duty-reserve-tax, esma-cfd-leverage-limits