My understanding is that there are minimum standards for brokerage firms have to enforce but they're overly lenient and most brokerages choose to apply additional limits to protect themselves and other things based on internal risk models.
I know for example that Scottrade imposes limits on margin maintenance levels based on prices. I'm sure there's an "adverse conditions" or "adverse risk" type clause in your contract with TDA that allows them to demand margin at different levels than they might otherwise.
So, short answer, yes, they're probably being jerks, but I'm sure its fully allowed in their contract. You'll have to review the fine print at TDA and see if they provide any further guidance on their policies.
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