This isn't a question about Surround Music formats, but i couldn't find a more suitable category.

I've noticed quite often that DVD movies have the DTS logo at the end of their credits, even if there is only the option to watch them in Dolby Digital. Why is that?

Most films are released with multiple formats on the print, because not all theaters are setup the same. Rather than have one print with DD, and one with DTS, and one with SDDS, it all goes on the same print.
http://upload.wikimedia.org/wikipedia/commons/thumb/1/1d/35mm_film_audio_macro.jpg/800px-35mm_film_audio_macro.jpg
At far left (and not shown, far right), outside the perforations, is the SDDS soundtrack as an image of a digital signal. Between the perforations is the Dolby Digital soundtrack (note the tiny Dolby "Double D" logo in the center of each area between the perforations). Just to the right the perforations is the analog optical soundtrack, with two channels encoded using Dolby SR noise reduction that can be dematrixed into four channels using Dolby Pro Logic. The optical timecode used to synchronize a DTS soundtrack, sits between the optical soundtrack and the image.(thanks, wiki)

This isn't a question about Surround Music formats, but i couldn't find a more suitable category.

I've noticed quite often that DVD movies have the DTS logo at the end of their credits, even if there is only the option to watch them in Dolby Digital. Why is that?

DTS utilizes a higher bitrate. Higher bitrate means more space is utilized on a DVD. On a long movie, that can mean that video compression needs to be higher to accomodate the soundtrack. There's also an SDDS logo as well, but you'll never find the Sony format on DVDs.

I'm not sure how much DTS costs to license versus Dolby though either.

There are different licensing deals in which the producers pay progressively more for each release format that uses a licensed decoding product i.e. DTS or Dolby. The first fee is for using the encoders which are technically leased or borrowed by mixing stages when making the film. Second tier is theatrical release in which if it is released in the format, you can have the logo in the film. More tiers are PPV, DVD foreign etc. which cost more money to use the format but may not have the logo removed from the print even if the format is not available on that format. In short, they could mix and release theatrically in DTS but save money by not releasing the DVD with DTS in order to save some money.

An interesting side note: SDDS (Sony Dynamic Digital Sound) is capable of up to eight discreet channels, but does not use the two extra channels in the rear, i.e. DTS-ES and DD-EX. The channels are instead left, left-center, center, right-center, right, surr. right, surr. left and sub. Also encoded as backups, are center, sub, left + left-center and right + right-center. These are included in case of damage to the print, resulting in drop-outs. The twelve channels use a 2.2 megabit bitstream(DTS theater version is only 1.536 megabit, but does not contain backup channels). The primary channels and backup channels are on opposite sides of the print, separated by 17 frames, in case of damage or a splice.
Sony Cinema Products packed it up in 2002, but the format is still alive and kicking as shown here. (http://www.imdb.com/List?tv=on&&sound-mix=SDDS) Seeing as how SCP is now defunct, that's why you won't see SDDS in the home. Also, home screens just aren't big enough.
That, and where the heck would you put the two extra front speakers? :D

This isn't a question about Surround Music formats, but i couldn't find a more suitable category.

I've noticed quite often that DVD movies have the DTS logo at the end of their credits, even if there is only the option to watch them in Dolby Digital. Why is that?

Many movies are released in theaters with DTS as an available projection option. Theatre DTS requires separate CD-ROMs that travel with the print.

When the DVD is made, it may only have a Dolby Digital track.