Is it normal for an instructor to refuse to tell students their current grade?
I've been that professor several times. Much to my own chagrin, there often is no such thing as "the current grade". On a typical day during a typical semester, there are scores for homeworks, quizzes, midterms and whatever other assessments have been completed so far, and there is a formula in the syllabus that computes a "final score" from all the assessment scores, but:


the formula assumes all scores to be known, not just the first few. Extrapolating is not trivial, particularly if the rules include things like "the lowest homework score will be dropped" or "later midterms will be weighted more" or "the first homework will be dropped if the next ones show improvement".
the cutoffs for the grades are rarely decided upon in advance; they often are determined by looking through students's work (the final midterm or exam is particularly good for that, being fresh in the lecturer's mind) and clustering students into categories (e.g., if you see someone doing really good work, you put that student into the A-cluster, so the cutoff for A will be no higher than their score). Some lecturers also curve based on pre-determined ratios (something I avoid, but I've heard of lecturers forced to do this by the admininstration), but again it is impossible to predict the final relative position of a student just based on their current status, as some students improve heavily during the semester.


So computing a "current grade" is a nontrivial exercise in forecasting -- and a thankless one, as the reward curve is biased to the negative (getting students' grades right will net you some thanks; getting them wrong will cause trouble all the way up to disciplinary action). Teaching is hard enough without it.