But the problem with caps, is that people hit them.

As a hypothetical, imagine a polluting plant poisoning the neighbourhood. You want them to know once their fine gets to $1M it's capped? You've suddenly presented a clear choice to the plant - fix it if it'll cost less than $1M, if it'll cost more keep ignore.

If you're going to fine anybody "per unit of time", you have to keep it open-ended (as otherwise the only people who'll ignore you, are those making more profit per day, than the per day cost)

It's not an all or nothing thing. You can cap some fines and not others obviously. Fining someone $573,000 for having vehicles in his back yard and working on them is ludicrous and absolutely makes the government look like either a bully, out of touch with reality, incompetent, or all of the above.