Canada decides not to be protected by US missile shield

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My title tells you what happened, a quote as to why:

Martin, who leads a tenuous minority government, has said Ottawa would not support what he called the "weaponization of space." Though he initially supported joining the program when he was a candidate for the Liberal leadership, Martin has retreated, since polls indicate that a majority of Canadians oppose it. Many believe that the umbrella, when fully implemented, could lead to an international arms race.

Let me repeat that: a missile shield will create an arms race. Maybe my logic is a bit off, but WHY THE HELL WOULD ANYONE MAKE MORE MISSILES THAT THEY CAN'T HIT YOU WITH?! Honestly, I can't understand the logic that would back up this decision. If anyone can explain it to me, please do. I don't believe it would cost Canada anything, Canada has a sucky military already. For example, Canada recently bought a diesel submarine, diesel. It was used during WWII, and took three years of repairs so that it could float (or, more accurately, dive only when it was intended to). It didn't even work; the engine room caught fire on its trip from the British shipyards to Canada, and one crewmember died. Diesel subs are useless and worthless nowadays, they have to surface once every three days or else fumes will kill the crew members, they are loud (and thus very easy to follow), they are cramped, they are old, and they are just all-around sucky. Nuclear submarines, on the other hand, can stay underwater almost indefinitely, run quietly, they have more room and are actually possible to live in, have modern equipment, and are quite nice - for a submarine. I think the US would practically give Canada a nuclear submarine for the price of its scrap metal. We'd be overjoyed that our northern neighbor has finally considered its own defense; and we'd probably save money in the long run because we'd need one less ship to defend Canada. Additionally, engine rooms don't catch fire on nuclear submarines.

...U.S. Ambassador Paul Cellucci told reporters he was perplexed over Canada's decision, which he said effectively allows Washington to decide what to do if a missile was headed toward Canada.

"We simply cannot understand why Canada would in effect give up its sovereignty - its seat at the table - to decide what to do about a missile that might be coming towards Canada," said the outgoing (Canadian) ambassador, who had vigorously urged Canada to sign on the plan.

(Canadian Prime Minister) Martin, ending nearly two years of debate over whether Canada should participate in the development or operation of the multibillion-dollar program, insisted his decision had not relinquished Canada's sovereignty over its airspace and that Ottawa would expect to be consulted what to do about any missile passing over Canada.

expect all you want, Washington won't call you over a missile. Washington would like to call you over a missile, it's the proper thing to do according to textbook diplomacy and not doing so would damage relations, but there are only a few minutes between the time the missile is over Canada and the missile caused part of the US to cease to be.
Simple facts for you: the defense system is US, it was built by the US, it was paid for by the US, it was the US that persevered to build the system despite all the bitching from Canada and the like, and it's damn well going to be the US that will decide what to do with it.
Here's the confirmation of that:

Foreign Affairs Minister Pierre Pettigrew, however, indicated the ultimate decision had always been in U.S. hands.
"Would it have been otherwise?" he replied when asked whether Canada's refusal to join means the country now officially relies on the United States for protection.