I think I can draw an analogy of sorts with a Pizza place.
Some such locations have a promise of delivery within a certain period of time; say, 30 minutes or it's free. a Promise of sorts- a verbal contract even. advertisements, etc display this new feature.
So, business picks up, because of this new promise. But guess what? because they were unprepared and are not beating the time limit often enough, they simply shrug it off; they'll tell customers "oh, it's just so busy" and so forth.
Well, the only REASON your so busy is you promised something that you apparently cannot deliver because you were unprepared for the load on your business to meat that demand.
This is probably a similar scenario to Comcast; people obviously choose their service based on a number of factors... one of these is the advertised speed. In this case- 15mbps. But because there are too many subscribers, or more precisely, because they are unwilling to devote more resources to improving their core architecture, they cannot always deliver this to all customers, and invoke esoteric rules that define that the service will be throttled back over 70%... in this case, that reduces the effective download speed from 15mbps (1.875 megabytes/s) to 30% of that, which is a mere 562Kilobytes/sec.
The fact is, all else being equal, the advertised speed should be the speed that it normally goes. If using this "advertised" speed for a certain unadvertised period of time suddenly reduces your speed to the same speed that can be had with far cheaper deals from comcasts competitors, exactly what do you gain?
basically, what you are signing up for is not 15mbps service, but rather 4496 kbps service that happens to go faster for the first 15 minutes.
As a comparison- I tested my connection right now:
the most interesting thing is, even with that speed, if I am downloading- say, a Linux ISO or some other very large file, then my transfer rate boosts to nearly 60mbps at intervals for a few seconds. This is ALL advertised by my provider and is EXACTLY what I signed up for, it should NOT be a coincidence that what I signed up for and pay for happens to be what I get. In Broni's case, what he is getting is FAR less then what he is paying for.