The real problem is not that some get charged and others get nothing, like ups and ems. Its the fact that ups has to be accountable to the government of the country it opperates in, and the customer. If a business is legit and they don't pay any duty on supplies from out of country, then they become tax dodgers. Read, on paper it is wrong to not pay tax (duty is applied to foriegn items so the government can make up for a non-taxed purchase) and so anyone allowing it or involved is bad.
EMS is just regular post. That ends up being handled by strung out postal employees of both the originating country and recieving. The CBSA checks out boxes and items that have not been marked as a gift. Some make it through deemed as "not-taxed" items even though they should, sometimes they under charge but rarely over charge. Its just how the parcel is marked, if it gets marked as a gift you probably wont get charged any duty no matter how obviouse it was purchase.