Considering that, I don't think they really care too much about a few engineers being allowed to keep loaner copies of software they used for testing.
Some countries have specific legislation to exclude air miles.
It would hit airlines who sell $$$$ worth of tickets each year, but then hand out discount vouchers/points/miles to customers (many of whom bought the original tickets on business expenses, but take the vouchers/points/miles personally, thereby evading tax). The tax authority would be able to estimate the 'value' in vouchers handed out (not the book cost or stated face value, but what customers value them at), the percentage of those vouchers that represent tax evasion, and then fine the airline 3x the lost taxes.
Same for starbucks loyalty cards etc.
As a law abiding citizen, I now have a moral dilemma. Pay tax as the law says, or don't pay tax because the government promised not to punish me?
Where will the latter lead? It's fine to do other crimes as long as you know the government usually won't prosecute? It's fine to write overly broad laws because we will only enforce them against the truly bad guys?
Perhaps it wasn't clearly articulated, but I see some moral dilemmas here.
1) Competing goods: obeying the law (civic virtue; collectivism) vs. personal happiness (hedonism; individualism).
2) Competing concepts of civic virtue regarding laws that won't be enforced: Is it better to (a) vigorously oppose such systems, because e.g. they lay the groundwork for tyranny, or (b) accept that some enforcement sloppiness is beneficial for various reasons, and should therefore be accepted?
On the one hand, you could argue that taking advantage of the IRS deciding not to bother with air miles is wrong.
On the other hand, if you decided to include air miles as taxable on your return, you might be obeying the letter of the law.
This reminds me of the central plot point of The Good Place, which I won’t spoil here, but will paraphrase indirectly: “at what point (if any) is it acceptable to stop considering the second- and higher-order consequences of our actions or inactions?”
I tend to hew toward your 2(b). If even the IRS isn’t going to bother, why would I if it won’t meaningfully matter?