Stepping back a second, at the most basic level a single fare is due for each and every journey.
However, to encourage patronage transport operators choose to offer discounts. This could be a return ticket, a daily fare cap, a season ticket, a through ticket between routes, etc. But each of these offers, just like a simple single ticket, comes with terms and conditions. Want the offer, abide by the terms. Pretty simple really.
As for "what's the point of tapping after reaching the cap?", well no doubt the operator wants the data. When they have all journeys correctly recorded they can decide whether the cap is at the right level, whether they might offer, say, a special evening rate, run extra journeys, all sorts of things. And in the Buses area of this board there have been several times where posters have been adamant that bus companies are "on the fiddle" because they claim drivers don't record concessionary passes and capped fares correctly. Journey records are important data.