As I've said previously, I found it funny that some of the same people who wanted to give Obama authority to bind the US in a deal with Iran didn't want him to have authority to bind the US to a trade deal. The opposite is also true.
I see no reason why Presidents ask for and receive such authority. Canada is involved in the same process and our government has provided some information related to the deal negotiations and the entire deal will be subject to parliamentary approval. We did similar with our deal with the EU which is currently in the ratification stages and with our deals with India and South Korea, to name two others.
Canada has no fear about opening up agreements with the world - it's the way of the future. If the US doesn't get involved, then the European and Asian markets will have free trade access to the US, through Canada and our NAFTA agreement, without the US having reciprocal access to those other markets.