I think this may be a matter of efficiency. NOTE: I am approaching this with somewhat limited experience, only having patronized restaurants in the US, and seldom gone higher than the average chain restaurant, so my perspective may not be the same as yours, but...

The fewer types of glassware a restaurant has to maintain, the easier it is for them to get bulk discounts on replacements, the easier it is to efficiently store and wash them, and the easier it is for the waitstaff to access them and get them to the filling stations and out to the customers. Ease of use speeds service.

Sure, alcoholic beverages still come in a variety of different containers, but those are handled by the barstaff, not the waitstaff. The waitstaff can simply pull 6 generic glasses out of the hopper and fill them with whatever - water, pop, tea - and they're done, saving them the time it may take to hunt for specific glassware and move around to multiple locations to fill it. It's a model that's been used at cheap restaurants for decades, and I think the sense it makes may be permeating the upper end restaurants.

I realize that some people will have preferences different than mine, but aside from personal preference, I really don't see any compelling reason for a restaurant to maintain separate sets of glassware for water, tea, pop, and any other non-alcoholic beverages, when a single set will do. What size, shape, and material they choose is up the individual restaurant, but there are definite benefits to larger sizes and plastic materials - larger sizes mean less frequent refills, which frees up a lot of the waitstaff's time, and plastic means fewer broken pieces in the washers and on the service floor. Plastic also means safer, because no one on the restaurant's staff will have to pick up potentially dangerous sharp pieces, and no customers will be exposed to potentially dangerous sharp pieces. A mop is all that's needed to clean up a spilled drink.

Plastic containers can also improve storage and staging; glassware often doesn't nest when stacked, meaning that water glasses have to be stacked the old-fashioned way, with trays between each layer. The stacks get high, heavy, hard to reach for shorter staff, and darn dangerous, not to mention difficult to move around from the washing station to the dispensing station(s). Plasticware nests, and is 1/4 the weight of glassware, so it's much easier to stack it safely next to the dispensing stations in easy-to-reach places.

All of this efficiency reduces the amount of time it takes for the waitstaff to get things done, which means that a) the same waitstaff can handle more customers, or b) the same number of customers needs less waitstaff - which translates to monetary savings and thus greater profits for the restaurant. Which they may or may not choose to pass on to the customers in the form of lower prices.

Right or wrong, agree or disagree, prefer or not, I believe that's what's behind the trend you're noticing with water glasses. It seems a small thing, but it's part of a larger picture, and that picture is constantly evolving.