in the wild it is instinct to host an anemone for survival...with tank raised fish it is different...they learn that there is no danger...if an anemonie is added it might host it or it might host something else...
I agree with the others that it's hit or miss when it comes to TR clowns...
My two TR Ocellaris (ORA) are now hosting in my BTA, but it took them a while to get the hang of it.
They started out hosting with a few inanimate objects, then moved on to my Goniopora, then a Pink Tree coral, and when I removed that, they moved into the BTA.
I think it's a combination of learned behavior and instinct... they have this 'need' for a host, but without other clowns to observe, they proceed by trial and error.
Even though there's no predatory 'danger' in my tank, whenever a powerhead kicks on from my wavemaker and surprises them, they both dash for the anemone and then check to see what it was that startled them... they're much more comfortable (and nicer to watch) since they've taken to hosting the BTA.
As for the ones that don't ever feel the need for a host, well... those are the ones that usually get eaten in the wild.
That's about the only explanation I can come up with...
I have a pair of TR Percs that hosted some barnicle shells for a long time. I put in an anemone at Christmas, and a couple months ago the female clown started investigating it. Only in the last week has she actually "laid down" in it. She will still go for the barnicle hole if startled, but it is progressing. I think it also has to do with something I read about TR clowns not having the chemical protection in the mucus layer, and that it needs to be built up over time. The behavior of my clowns would certainly support that, with the slow careful introductions.
I think it fired cytoblasts(?) that accumulate in the mucous coating that causes the anemone to eventually recognize its own signature as it were and stop treatin the fish as a threat , at least this is thought to be part of the process, FWIW