Why is American English so wedded to the subjunctive?

In the sentence 'She suggested that they go to the cinema' there is no way of telling from the sentence in isolation whether it means that the speaker gave advice on attending a moving picture show, or whether the speaker believed that some people were already in the habit of enjoying cinematographic entertainment. The same is true in relation to an individual when the past tense is used, as in 'She suggested that he went to the cinema.' We depend on context to tell us which meaning is intended.

Now, when we come to the third person singular in the present tense, I understand that American English distinguishes between the mandative subjunctive ('She suggested that he go to the cinema') and the indicative ('She suggested that he goes to the cinema') to express the two meanings. Why does American English insist on an inflectional distinction in the third person singular here when it is obliged to rely on context elsewhere?

Answer

(this is totally "armchair philosophising" - I've no authorities I can cite to back me up here)

I think there's a tendency to assume AmE (American English) is somehow "more advanced" than BrE (British English), simply because their orthography got a major revision from Noah Webster's dictionary at a period in American history when such wholesale changes could actually succeed.

In many respects though, it seems to me AmE is actually more conservative than BrE. I believe this stems from the fact that over the past couple of centuries, a significantly higher percentage of Americans didn't have English as their mother tongue (I think it's now higher in the UK, but I can't find a source to back me up even on that).

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So turning to OP's question, I suggest we're dealing with something akin to the hypercorrection whereby a Cockney - knowing he's prone to drop aitches - might sometimes add an aitch that shouldn't exist, when trying to speak "correctly".

By the same token, a disproportionate number of Americans might overzealously apply fine/meaningless distinctions because they don't have the confidence not to (lest they be mistaken for non-competent speakers).

Again, with no references, I believe there are more "autonomous, distinct dialects" in Britain, despite the much lower population (many such dialects are now actively promoted by national media). The net result may well be that competent speakers in Britain are more comfortable with discarding "dated" inflections and other language variants which in practice rarely lead to ambiguity because context normally make the meaning clear.