Side Fee Fallacy

There is a theory that off-chain fees are illegitimate. The theory holds that a merchant paying a miner "on the side" to confirm the merchant's transactions prevents other merchants' transactions from being confirmed, or that it raises the cost of those confirmations.

One impact of such arrangements is that an average historical fee rate cannot be determined through chain analysis. The apparent rate would be lower than the market rate. This could of course lead spenders to underestimate a sufficient fee. However there is no aspect of Bitcoin that requires future fees to equal some average of past fees. Estimation would necessarily compensate. Ignoring "free" transactions in full blocks would be a reasonable start.

Another impact is that disparate relative fee levels can highlight certain transactions as being associated with such arrangements. This can contribute to taint of the merchant's transaction and/or the miner's coinbase. But given the arrangement is a choice made by the creators of these transactions, there is no privacy loss.

There is no impact on market fee rates or the ability others to obtain confirmations. If the arrangement deviates from market rates then either the miner or the merchant is accepting an unnecessary loss. This is no different than the miner confirming transactions with below-market on-chain fees or the merchant overestimating on-chain fees, respectively.

Furthermore, unless the miner's hash power is 100%, the merchant is increasing his/her average confirmation times by paying a side-fee at what is otherwise a market rate. This implies that the market rate for side fees is the product of the miner's hash power and the prevailing market rate. In other words, proportionally less than apparent market rates. Without understanding this effect one may misinterpret the actual effective rate in such arrangements.

Bitcoin provides a mechanism for on-chain fees so that a transaction can compensate any miner without having to know the miner's identity. It is a privacy-preserving convenience. If miners and merchants want to weaken their privacy by performing additional tasks, nobody else has any basis to consider that illegitimate. This theory is therefore invalid.