The fact that similar things are analogous or have similar effects doesn't mean they are the same thing. The purpose of that massive collection of words in the

The fact that similar things are analogous or have similar effects doesn't mean they are the same thing. The purpose of that massive collection of words in the unabridged dictionary is to draw fine distinctions. For an example, trade tariffs and quotas are similar in almost the exact same way you describe here and have very similar effects. However the distinction is in how the financing for the costs works. Who pays? How directly do they pay? The answers explain why tariff isn't a quota. In your example as well, you change a payment from a direct monetary one to a diffuse externalized cost, thus creating a distinction. The language has evolved to capture that distinction, your desire to conflate the two notwithstanding. A subsidy is a direct monetary grant.