Actually, instead of "value" I probably should've wrote "price" in the original post. The price never falls, and rises whenever new people buy BONS. This is a play on the difference between current price (related to market capitalization), and the actual price you'd earn for selling a large numbers of tokens (related to liquidity), where the latter is always smaller.
For tokens with low liquidity, this discrepancy is larger. Selling a small percentage of all tokens may reduce the price by a large percentage. BONS is the extreme case of this, where selling any amount of tokens reduces prices 100%, i.e. you can't sell any tokens for any non-zero price. Indeed, you can't sell your tokens at all.
A class of tokens where this discrepancy can be neatly calculated is tokens based on bonding curves. These coins compute their per-token price based on a formula with the total supply as an input variable. If the bonding curve has the form f(supply) = some_constant * supplyexponent, then the market capitalization is always exponent + 1 times bigger than the amount of money poured into the token, so the token has seeming created money out of thin air, just by minting and burning according to some simple price setting function.
BONS is crudely based on the linear bonding curve f(X) = 0.01 * X + 0.000001 X where X is supply. If the BONS contract allowed burning, its "market capitalization" would artificially be 2x higher than the amount of POL that would be stored in the contract.
Now of course, most tokens aren't based on explicit bonding curves, but I'd imagine market forces roughly follow similar principles, with real liquidity being proportional to and substantially smaller than market capitalization.
Do however note that market caps can be higher than liquidity for legitimate assets too, for instance the US stock market is often valued higher than the entire US GDP, see https://en.wikipedia.org/wiki/Buffett_indicator. The difference is that stocks and useful cryptocurrencies create (add) value, and investors earn money from customers or blockchain users, and are therefore justly valued above the initial investment. Here reluctance to provide infinite fixed-price liquidity is based on "what does the seller know about this assets value that I don't?", not "people are selling, so I won't be able to resell my newly bought tokens to any other invesors for a decent price". The latter case is zero-sum, and taking a linear bonding curve coin as a simplified model, the average investor will only be able to sell their coins for half of the current price used in computing market capitalization. This predictably leads to rushes to sell these coins before everyone else (as those earlier in the queue get a better price), unlike useful stocks or cryptocurrencies, where low prices mean a great time to buy.