They aren't building a wall, they are simply sending a signal that the value you are currently taking advantage of is higher than you initially paid for it - and we realize it.
I see nothing wrong with the way they have done it. If they forced you to increase your monthly spend as a current client, then that would be more akin to your scenario. But they aren't doing that.
A better analogy would be, they have a nice lush garden setup with fruit stalls everywhere. As one of the first 100 people into the garden, you get all you can eat for $10. However, for everybody after that you have to pay $20. If you know this, and leave the garden and want to come in later....well, whose fault is that?