Normally on the offer page there is a graphic at the top which identifies which destinations are valid if you click on your departure city. I have not seen an offer yet which shows any destination other than YHZ as valid from YTZ. It also would not surprise me to know that YTZ is a valid destination from YUL and the revers is not.

I agree with you that this is pretty anoying since it forces me to spend $40 on parking when I could otherwise walk to the airport...and by the same token I've also been anoyed that while EWR is a valid destination from YYZ, LGA is not which prevents me from taking advantage of the code while routing via YUL.