The *only* reason to offer some sort of price promotion later instead of just giving you the discount up front and making you sign a contract (even for three months of service) is so that they can roll the dice on customers signing up after being enticed by the promotion, and then not claiming the promotion. Rebates work the same way.
Partly agreeing with others here have said, I can think of two other reasons. One is to get customer information. The other is to hold money a little longer, so that there is more money available for the company to use.
Residential broadband networks were never engineered as video delivery systems.
Damn the cell phone networks for needing to be upgraded to accomodate people who quit using landlines!
After all, serifs were inherited from Roman techniques for carving letters into giant stones, not in writing: I doubt Roman sculptors would have added things that seemed to decrease legibility to monuments.
I can think of some analogies between chipping away at stones and pushing pixels. Not the least of which is that I it often seems just about as tedious.