Not sure what the logic is behind the assertion you should only have US stocks in an RRSP?

The only relevant factor of when it comes to an RRSP or any registered account is the tax implication.  Generally speaking you want to avoid holding stocks or investments in an RRSP if upon withdrawal the money will be subject to a higher rate of tax.

The only difference (for your typical investor) between US stocks and Canadian stocks in this regard is the taxation of eligible Canadian dividends.  So while it may make more sense to keep Canadian dividend stocks in cash account or TFSA that doesn't equate to saying your RRSP should only hold US stocks.  It really depends on your current and expected future tax rate, as well as your overall portfolio composition.