The OP should be congratulated for bravely pointing out, in layman's terms, what is termed in the discipline of economics as "microtransactions".
Microtransactions are prevalent in online gaming or video games.
A microtransaction is a business model where users can purchase virtual items - such as improved weaponry or additional lives to extend game play - for small amounts of money. Microtransactions often appear in free-to-play games, meaning there is no cost to download the game, just the cost to buy the online virtual products.
Game developers have learned to take advantage of this new revenue source. Chess.com - with its army of programmers, data analysts, accountants and gaming lawyers - surely is no different in this regard.
In the case of Chess.com, the main microtransaction - purchasing Premium membership - provides you with a subtle yet incredibly useful benefit: slightly easier opponents.
Not purchasing membership, inversely, almost guarantees tougher opposition in your rating range. This includes opponents of questionable ethics e.g. sandbaggers, cheaters, smurfers, GM speed-runners, etc.
transference/accountability issue.