how can landing the plane harder than usual be good for the aircraft
Define 'usual'. And stop comparing a RYR 738 with a BA Airbus - they are different aircraft. A Dash 8 lands differently to a 146. A 747 lands differently to an MD11. Do all cars handle the same and have identical suspensions? Note also that identical aircraft may land differently according to circumstances. Give me a two mile long runway, 10kts of wind straight down it and no other traffic, and I might deliver what you think is a good landing. Give me a shortish runway, and/or other traffic waiting to depart or land behind me (i.e. 9 times out of 10 for most of us) and you will get a positive arrival, possibly firm - you might think it hard - but it will be safe.
That is usual.
It seems normal to me, how can landing it hard and planting it down be normal?
Your idea of a 'hard landing' and mine are very different. You are basing your opinion on limited knowledge of the situation. As I have already explained, what you as a passenger may regard as a 'good' landing may be regarded by the pilots as a poor one.
Well quite. You have, however, had the benefit of several professional pilots telling you the same thing, that landing a large and very fast moving aeroplane is a complex task that involves an awful lot of variables, and that with the best will in the world, you as passenger are not best placed to judge what constitutes a good landing. I apologise if I'm starting to sound tetchy, but while most of us on Pprune are happy to answer passengers' and enthusiasts' questions, it gets a little wearing when people appear unwilling to accept our explanations.
If you really want to appreciate what's involved, book yourself an hour in a 737 simulator. There's a very nice one down at Bournemouth - it's a -200 series rather than an -800, but it will serve the purpose. Practice landing it. In the unlikely event that you find it easy, get the instructor to dial in some crosswind or turbulence. You might find it a bit of an eye-opener.