Here's a couple of questions that might help you put this into perspective.
- Do you normally expect your students to have read (in at least cursory fashion) the textbook material the lecture is on before or after the lecture itself?
- Do you provide a calendar of expected lecture topics, and expect this to be useful to them in advance?
- Do you view your job in this course as one of introducing new concepts, and then leaving it to them to come to grips with these and how to use them through homework? Or of one of helping them understand and apply new concepts?
Now ask yourself how the lecture slides relate to your answers (which may differ depending on the course). If you are already expecting them to have a certain familiarity before you lecture, then what is the distinction that says that providing the slides before you lecture is bad but insisting they read the textbook is not? You don't even need to insist on it, really; the text is already there and they can always read it ahead of time, even if it's not expected of them, same as they could with lecture notes.
I've known a number of mathematics professors and lecturers who feel that the amount of lecture time available is inadequate to fully and properly explore the required topics to their full satisfaction. As such they must make certain concessions. One of the typical ones is to tell the students that they are expected to have performed a basic reading of the material in the textbook prior to the lecture on it. This lets the instructor focus a little bit less on trying to set up all of the new concepts, and leave many of the proofs to the text, and spend a little more on getting them accustomed to using it and understanding it via examples. Especially so in lower division courses such as calculus, where applying it is of greater importance than knowing how to prove it; but rarely so in their graduate courses, where the concepts and proofs are often of much greater importance.
That said, I've not seen even amongst those a consistent philosophy on the provision of lecture notes/slides. Sometimes they object because their notes are not written in a way to be useful to students (only to them), sometimes they object because they think (some of) the students will be lazier if given them, sometimes they provide them slightly before, sometimes long before, etc. And I've not seen any consistent correlation suggesting one approach is better than another; either in student achievement or in student reviews.
You need to figure out what goals your institution expects you to achieve with your course, what your philosophy of teaching is, what it is that will make you the most comfortable and effective at teaching, and how these things play into and impact your decisions. And, of course, always be willing to learn from experience should things not work out as well as you'd planned!