Body of Law:
FINRA Rule 4210 is a messy one. It is complicated and difficult to read, is subject to interpretations, many of which have not been updated in decades (e.g. the published version on the FINRA site has not been updated since 2010), and has been supplemented by margin requirements imposed by FINRA through regulatory notices such as this one, under authority pursuant to FINRA Rule 4210(f)(8)(A). That provision permits FINRA to prescribe additional requirements "whenever it shall determine that market conditions so warrant."
As a process matter, it is difficult to see where FINRA has made the case that "market conditions" merit this change. There is less than a page of explanation, and little to suggest that there are any particular conditions in the market, as opposed to a basic disfavoring for a type of product. "Market conditions" also suggests something temporary. But there is nothing in this Regulatory Notice to suggest that these changes are temporary, and FINRA has made a habit of making "temporary" changes to its margin requirements through regulatory notices. See, e.g., Notices 08-08 (Auction Rate Securities), 08-60 (Money Market Mutual Funds), 09-53 (Non-Traditional ETFs) and 11-16 (Treatment of Non-Margin Eligible Equities).
This is not to say that there isn't a case to be made that ETNs should be subject to higher requirements. It is correct that ETNs can be of a different nature than typical corporate debt. But that would seem to be something that could be addressed through a rule change or, at worst, through a temporary increase followed by a formal rule change. Further, FINRA provided no lead time (other than a request as to an "undue hardship") for firms to adjust their margin systems or to reach out to affected customers. A customer with positions in ETNs could see their margin requirements go from 3% to 25% overnight.
Finally, this regulatory notice also does not make distinctions that otherwise apply in Rule 4210 as to the type of customer. The regulatory notice applies the new margin requirements to all accounts, regardless of whether they are "exempt accounts" under Rule 4210. For many other types of non-junk debt, the rule provides differing treatment (generally with lower requirements and, in some cases, an option to take capital charge) for transactions with exempt accounts.