Heres the point I don't fathom. I assume that you have no problem making legitimate purchases of titles if those previews didn't waste your time?
And clearly you (by your examples) use technological means to download (what can be 50 gigs to match the quality of a Blu-ray feature) large amounts of data, store said data and then play said data. This requires (I would assume) high speed internet (how much does that cost?, how much does downloading an equal quality product require you to alter what you normally would use the internet for if the blu rays removed that feature), and of course it does require some time to do.
So clearly this option has certain costs to it, set time issues, and the alteration of normal patterns of your internet use.
If this is "the" reason you bit torrent for bluray quality films, then why not just buy them and rip them your self?
As your point is clear you can absolutely remove that on your own. You can create those same files. Hell you can do it automatically as easy as it is to download.
In fact in my house its easier. Less steps. Put Disc in, drag icon to program and in 5 steps that takes about a minute program does the rest, let it go for the night and low and behold here is my file. It takes me longer and more steps to find and locate the bluray file on a torrent site, verify its status (and its quality) let it do its business, then the option listed above. In both cases the time intensive processing (or downloading) can be done either when one sleeps or in the background while you perform other tasks.
And it gives you the full options for languages, bonus features, and low and behold even the trailers if you are so inclined.