Its because there are subs present, it hard codes the subs into the video, PS3 doesn't support ASS or srt or txt or vobsub or whatever.
Unless you export the subs into those dvd or avchd image subs and re-construct the IFO, and use Sony's BD author or DIY the AVCHD structure yourself and done!!
Its not that complicated, just needs some practice snd getting used to, soon you'll be authoring home dvds sooner than you've ever thought of.
Go to Doom9, they've got some amazingly detailed guides ready for you.