I'm a former broker on Bay St. It's not worth their while. The number of Cdn accounts are small Some of the US firms have been caught soliciting in Cda, can't do that, unless you are registered. If you are that concerned about your money, don't get in the market. There are all kinds of risk in trading.Trade with the banks and pay big fees.
If you sign an Non Solicit form that takes care of the legal stuff. You are going out and finding the broker on your own.
This was a gray area... Until the OSC sued clearing firm Southwest Securities circa 2001-2... And kicked Southwest out of Canada. Since that court case... It's been CLEARLY ILLEGAL for any US broker to do business in Canada... Unless they open a branch office in Canada that is regulated in Canada. This applies to all stock trading for sure... So any US broker that is taking Canadian customers... Is knowingly breaking the law... And is probably a small, shady outfit... But then if you only have $10K... who cares. Also, Portfolio Margin is illegal in Canada. The end result is that IB is pretty much the only game in town... For high volume Canadian stock traders. There are ways around this... Like simply forming a US Corporation.
What you are saying might possibly apply to Stock Brokerages, but definitely not to Futures Brokerages.