The regulatory answer is that the wrapper does not care what the manager is doing. Rule 6c-11 provides exemptions for both index-based and actively managed ETFs and, in the SEC's own words, "will not by its terms establish different requirements based on whether an ETF's investment objective is to seek returns that correspond to the returns of an index". The reason given is operational: index-based and actively managed ETFs that comply with the rule's conditions "function similarly with respect to operational matters, despite different investment objectives or strategies", because both register under the Act, both issue and redeem shares in creation unit sizes in exchange for baskets of assets, both list on national securities exchanges, and both let investors trade through the day at market-determined prices.
The separation that used to exist was history rather than principle, and the SEC says so. The distinction in the older exemptive orders "is largely a product of ETFs' historical evolution", because the SEC "did not approve the first actively managed ETF until nearly 15 years after index-based ETFs were introduced". Since 2008 the market grew considerably, the SEC watched how these funds operated, and it "has not identified any operational issues that suggest additional conditions for actively managed ETFs are warranted".
Daily portfolio transparency is the condition, and it is specific. Rule 6c-11(c)(1)(i) requires an ETF, each business day and before the opening of regular trading on its primary listing exchange, to disclose prominently on a free public website, for each portfolio holding that will form the basis of the next net asset value calculation: the ticker symbol, a CUSIP or other identifier, a description of the holding, the quantity held, and the percentage weight of the holding in the portfolio. Those holdings must be the fund's holdings as of the close of business on the prior business day. For a manager whose value is supposed to lie in what they have worked out before anyone else, publishing the whole position list every morning is a real cost, and it is the price of using the rule.
That cost is exactly why the non-transparent category exists. In 2019 the SEC granted an exemptive order permitting certain actively managed ETFs to operate without being subject to the daily portfolio transparency condition included in other actively managed ETF orders. The release records the consequence in one sentence: "because these non-transparent ETFs do not provide daily portfolio transparency, they would not meet the conditions of rule 6c-11". They are not a milder version of the same thing; they are a separate arrangement operating under their own relief, and the trade-off they make is protection of the strategy in exchange for showing holders less.
What does not change is worth listing, because it is most of the product. The creation and redemption machinery, and the arbitrage that keeps the market price close to net asset value, work the same way regardless of what the manager is doing, and depend on the same authorized participants. The bid-ask spread is still a cost of every trade. The in-kind mechanism that makes the ETF wrapper unusually tax-efficient in a taxable account is a feature of the structure, not of the strategy. And the question of whether active management earns its fee is entirely unaffected by which wrapper it arrives in; our page on active management sets out the evidence, and the arithmetic of the cost difference belongs with index funds. A cheaper wrapper for an expensive strategy is still an expensive strategy.