I believe it's because the management bonus/incentive/promotion system is primarily focused on:
1. Deciding on the right product to build
2. Making more money off the product than the cost to build it

Notice that "building a quality product" is not in that list.

Many companies can't claim success on #1 and #2 more than 20% of the time, so that gets the majority of management focus.
This is because a manager doesn't get a bonus on a super well-engineered product that no one wants to buy.

Engineering quality often doesn't get noticed/rewarded until AFTER the product is a success. Which often is less than 20% of the time.

Many families can easily watch 3 movies on a weekend. Split over Friday, Saturday, and Sunday, three movies isn't that many, especially if you don't have cable TV. Multiply that by 4 to 5 weekends in a month, and you have 12-15 rentals per month, which netflix considers to be a "heavy user". Shouldn't people who pay for "3 movies at a time" be able to watch three movies a week?

That said, the 3 movies at a time plan is a great value, if only for convenience. If netflix is losing money with a 3 movies a week usage pattern, perhaps they should charge more, rather than throttling. I think legitimate users would be willing to pay a little more. The problem is, the "three at a time" plan is the best plan neflix offers.