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Huh? How am I making excuses? We're talking about MPEG Streamclip not being able to OPEN H.264 material, not what it outputs. It can't output anything if you can't even open the file in the first place...

Toast Titanium can open/convert .ts files, then you convert to your editing codec of choice and edit in iMovie, Final Cut, whatever. However, I think the original poster is looking for a simple trimming solution for cutting out commercials, presumably without having to transcode formats.
 
Use ffmpeg in Terminal to put the .ts into a .mp4 container then use Quicktime to trim. Using the arrow keys is a handy way to move frame by frame and command+Y is handy to split and then delete the unneeded section.
 
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