Well, it is a premium plan - vote with your wallet
Nobody is forcing anyone to get the 100Mbps tier. It is a premium tier for those who really want it.
I fully understand folks who have concerns that there should be affordable Internet access (which is an endlessly debatable term - at one time, I thought having a 1.5 Mbps dsl line at $34.99 a month was a great deal), so that this is widely available to most everybody. Broadband access may not exactly be a "right", but I am comfortable that it has reached the level of being a basic "utility" for most folks, and access should be reasonably available and affordable for a base rate of performance.
I see no rationale that Charter, or any other ISP has got to offer any "justification" for their pricing of higher-level tiers other than whatever they feel the market place will sustain.
Although this statement will not be correct in perpetuity, as of right now, nobody "needs" 100 Mbps service.
Don't like the price? Vote with your wallet and don't get it. It is pretty safe to say that Charter needs to hit a certain threshold of customers on their 100 Mbps tier for it to be profitable - let the market place speak.
Deeds, not words