Isn't this cabotage?
 

Looking to maximize mileage in a year where I will just sneak by the status threshold. Why am I even being offered this routing? Isn't this illegal for me to fly?
http://i.imgur.com/Auutu.png
YOW-DCA-PHX-YVR

IMO, this is not cabotoge. Now, if you were to fly YOW-YVR nonstop on US, that would be a different story. :D

I'm pretty sure it is Cabotage but I don't think its illegal for you to FLY, just illegal for them to offer it.

From Wikipedia
Cabotage situations can also occur as a consequence of hub-and-spoke operations. Consider that Air Canada has a major hub at Toronto that offers flights to several U.S. cities. While a passenger is able to buy a ticket from Boston to Toronto, and a separate ticket from Toronto to Seattle later that same day, both flights cannot be offered on the same itinerary because this would effectively be a U.S. domestic service

Good point. However, the OP is flying outbound from YOW to DCA on a codeshare partner, which I would imagine to be AC. Then DCA-PHX-YVR segments are on US metal with two different flight numbers. So I wonder what the cabotoge definition is when you get into the weeds like this. The Wikipedia definition is not clear.

YOW-DCA is Air Wisconsin dba US Airways Express. All three flights are operated by United States metal.

Is this one fare or a combination of two fares?

Most likely 2 different fares, which makes it legal

Definitely two or more fares combined. AC and WS are the only carrier that can sell YOW YVR fare. None of the US airlines have freedom rights to do so.

I can go BUF YVR or YYZ SEA/BLI but not YYZ YVR on any US airline on one ticket.

My understanding is that Air Canada offering a USA to USA trip is cabotage. And a US airline offering a Canada to Canada trip is also cabotage. So if the OP is on one ticket on US originating and ending in Canada, then it's cabotage.

If, let's say, the carrier was Air Canada and it was something like YOW-PHX-YVR, then it wouldn't be cabotage, but AC doesn't offer domestic flights with a US connection because it isn't in their, nor their passengers' best interests (mileage runs aside :) ).

If I'm wrong on this, please educate me. :)

If you buy a YYZ-WAS-YYZ trip that's not cabotage that's a round trip. Here the OP's buying YOW-DCA-YVR, which is an open jaw. That's legal as it's 2 piecees of one-way fares. If the airline offedred YOW-DCA-YVR as a single fare it would be illegal.

Nowhere did I mention a round trip. Obviously round-trips are not cabotage.

I'm saying YOW-DCA-YVR on a Canadian carrier wouldn't technically be cabotage, but on a US carrier it would if it didn't include a full stop (i.e. open jaw trip).

The OP's trip is not an open jaw. It's a one-way trip with two stops. Perhaps it really is two one-way fares pushed together in a way that is transparent to the buyer.

Flying YOW-DCA-PHX-YVR is not cabotage. YOW-DCA-YVR is.

Flying SEA-YYC-YYZ-GRR-YYZ-SEA is not cabotage.

Neither is SEA-YYC-YYZ-GRR-SEA.

Point is, domestic segments such as DCA-PHX and YYC-YYZ render cabotage concerns as irrelevant.

Air Canada's website will gladly sell you a USA1-CAN-USA2 multi-city ticket (i.e. SFO-YVR-LAX) with both segments on the same day. You are purchasing separate fares so I don't see anything wrong with this. A problem would arise if it was one fare basis.