Think of it more as systemic risk - I would say the general logic error I'm seeing on HN is assuming all assets on deposit are firewalled, have completely uncorrelated risks, are attributable to exactly one type of funding, either margin or deposit, and information is instantly communicated - in practice these are big messy orgs with big messy books, and the lender is looking at robinhood saying "you're asking for a historically high line of credit because you're on an exponential growth curve for users, whose assets are based on 1, generously 3, meme stocks? Let's try to figure something out here, we don't want you to shut down, but we need to mitigate risk"

People are expecting more a scenario in which clients stocks are neatly divided in two piles based on an unrelated criteria, i.e. if they are attributable to margin, or whatever assets you have on deposit. Additionally, the process of having margin is itself like having a credit line - AmEx simply doesn't have the systems or procedure to lower your credit limit because Tickle Me Elmo speculation is at a all time high over the last 72 hours, and it sees you're purchasing tickle me elmos, getting assigned a credit limit is a function of your credit score, and amex isn't tracking line item level purchases