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?

(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).

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.

You may as well ask “why” the Romance tongues have also preserved the mandative subjunctive. Certainly one possible answer is because they (and we) find it a useful distinction to respect and apply. However, one can easily devise many other potential explanations, all with no clear way to choose between them.

Similarly, you may as well ask “why” French has switched the protasis of a conditional to imperfect indicative when the other Romance tongues retain the original imperfect subjunctive for the same. Or per the discussion the other day, “why” British people seem more tolerant of ought not to do where Americans more often use a real modal and say ought not do. Or “why” a British person is wont to use a bare do in places where an American needs must supply a proper complement and say do so, lest it come off as ungrammatical. Or “why” British say different to where Americans say different from. Or “why” the English (but perhaps not the Scots) prefer proved where Americans use proven.

All these matters of grammar are quite different from something like definitively tracing spelling changes back to Webster. There need be no “why” for any of these; there is just “is” and “does”. All you can do is document that they occur. The discussion may be interesting, but I cannot see it drawing towards any conclusion. I honestly don’t see how this “why” answer can have a single, testable answer.

I’d be be delighted to be proven wrong.

English is a great language in many respects, but it isn't perfect. Many people believe that operas sound better sung in Italian, for instance.

Perhaps a better question would be: "Why don't we have the availability of more inflectional distinctions when using verbs, with which we could more easily avoid ambiguous interpretations?" (I fail to see any pure Americanisms in the examples given - they all sound normal, if not conversational, to my British ears.)

There are, however, convenient ways to avoid the ambiguities - for instance:

She suggested that they should / might go to the cinema.

She suggested that they were cinema-goers. ('they' could of course be meant to include the speaker - further clarification might be required)