Not a stupid question at all tax man. The brokers have buried deep down in all the legal micro print that they reserve the right to limit trading on any security they choose for ANY reason...and they don't have to give you a reason. Now if one takes a look at their agreement with their broker, it also states that one is COMPLETELY RESPONSIBLE for any and all trades they allow through their account. So given that, it appears the brokers are contradicting their own rules.
I look at it this way...TDAmeritrade solicited my business. I pay them a fee to transact my wishes. It is NOT their decision how I choose to spend MY money. Their responsibility is to transact my buys/sells...for an agreed upon fee...period. If however the buy or sell does not transact, that is out of their control.
Now, if they choose to limit buying of a security that I choose to initiate a buy order for, I would fully expect them to give me a valid reason as to why they will not put in a buy order. If it is a stock they have uncovered proof, or valid suspicions of fraud, wrong doing...you know the routine...then I would applaud them for looking out for my best interests.
But to arbitrarily limit a security and explicitly refuse to give a reason...other than..."we think something is suspicious" doesn't cut it. We are all adults...and expected to have enough common sense to handle our own affairs.
I contracted with them to buy and sell securities for me, and I pay them to do just that. That is what they solicit their business as...and not a guardian of my judgement. I do NOT pay them or ask them to make my decisions for me.
I get a little ticked off on this subject, so don't take the rant any other way than directed at the brokers.
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