Each customer gets a discount once - when they initially sign up. So for example for your account, the offer you got in 2013 was a month of free service, a $39.95 credit. Offers vary over time, sometimes they're a month or two free, sometimes they're a monthly discount for a period of time. But each is a one-time event - we don't provide new customer offers to existing customers.
Part of this is business viability (we couldn't keep going as a business if we reduced everyone's price), part is marketing (it takes a compelling offer to get someone to switch from another service), and part is fairness to our members overall.
The last point is key: it is typical in the industry to offer re-rates and save offers to SOME customers, the ones who complain or ask for it or threaten to cancel if their price isn't reduced. We don't feel that this is fair to ALL of our customers, and it also doesn't make for a great place to work for our staff. Instead, we have a one-time promotional offer for each new members, but then all of our members get the same terms ongoing.
There's another layer to this concept of fairness: if YOU get a lower rate, to keep the business online, someone (everyone) else must pay more to make up for that. That's a bit corrosive to a community of members with a common goal: fast, reliable service with improving technology and great customer care. Those things have a cost, and it is not reasonable that those who complain pay less while those who do not bear the resulting higher cost.
I hope this makes sense, and seems reasonable to you. These values are something we've certainly thought a lot about over the years.