Well I am a Canadian and do invest in the US market in a small way.
I am not allowed to invest directly in the US market through a US broker discount or not because I don't have tax information, specifically a social security number.
Similarly though Canadian exchanges have the highest listings in the world for Oil & Gas companies and Mining companies, Americans cannot directly invest in the Canadian markets....it is a TAX restriction to do it directly for an individual.
http://www.tmx.com/en/pdf/Mining_Sector_Sheet.pdf
http://www.tmx.com/en/pdf/OilGas_Sector_Profile.pdf
Unlike American brokerage companies, Canadian brokers provide direct access to American stocks of any stripe. These brokers look after the payment of taxes in the USA and give the Canadian investor the tax details to be used on our Canadian tax returns.
So we have the ability to have unfettered access to American markets including the OTCB.
Since there is that access to American markets, US companies don't need to have a duplicate listing on Canadian exchanges in order to attract Canadian investors.
On the other hand though, American brokers (unless you get a more expensive Global brokerage account) do NOT provide access to Canadian markets...which limits American investors to a lot of Mining and O&G companies...especially start-ups. They do have access to Canadian energy and mining companies that decide to go through the expense of listing on an American exchange in addition to their Canadian listing.
Actually a lot of the less expensive stocks and some expensive ones (Wajax Industrial trading at about $36/share) use the OTCB to gain access though most US investors think everything in there is junk.
So, yes, Canadians (and I expect other nationalities) have access to USA exchanges through our brokers (discount or not). The broker pays the taxes on our trades when the Buy/Sell is complete, subtracting those taxes from the final total trade....they then at tax time provide the tax information to us at tax time.
I do note the taxes deducted at the USA end are rather low but then we pay the bulk of the tax for capital gains in Canada...probably as mentioned by someone else because there are agreements between Canada / USA as to how such income is taxed.