Curry is taking about 12 threes per game against the Celtics, which is comparable to his season average. And the Celtics haven't played him the same way all series. Most notably, they dropped so far off him in Q1 game 1 so as to just be leaving him open, but they have generally not been dropping that far all the time. Third, the Celtics halfcourt defense has generally been good overall, where they have struggled is mostly in transition.
Is "deny threes to Curry" a viable halfcourt defensive strategy? If not it makes sense to tend towards letting him shoot more difficult ones.
If the result of drop coverage was Steph doubling his attempted threes per game, or somehow letting him take higher quality threes, then it would be a problem. So far that does not appear to be the case.
Also worth noting that, in this series, his usage rate (34.0) is up a bit over reg season (30.8) and playoffs (31.9 including finals), while his assist rate (20.6) is down from regular season (30.4) and playoffs (28.1 including finals).