TDA still allow orders, just not online orders.
They are brokers only, they don't make markets in any stocks, they don't trade their own account, so their is no benefit to them in restricting trading.
They take orders from customers, send them to market makers or ECNS to make the trades on their customers' behalf, and charge a commission for the service.
That's it.
When they restrict buying or selling, it's just online orders, you can phone orders in and still be charged online commissions. They do this so they can get verbal assurance that customers have not been solicited to buy the stock.
In their own words:
"Ameritrade is a self-clearing, online brokerage firm offering self-directed brokerage accounts to a national investor base. As an online broker, Ameritrade does not solicit orders, make discretionary investments on behalf of clients, or provide research or advice regarding securities. Rather, Ameritrade accepts trade orders to buy or sell securities and delivers those orders through it's clearing division to the appropriate market, market maker, or Electronic Communications Network ("ECN") for execution, in exchange for a moderate commission. Ameritrade does not trade on it's own account or make a market in any security. Client trades are executed on independent exchanges, through independent market makers or through ECN's."
Market Data 
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