Why don't they show subtitles for the Spanish dialogue?

It's a deliberate directorial device. We're experiencing the world through Walt and Jessie's eyes, so it allows us to appreciate their confusion, suspicion and fear through the powerlessness they feel when events are unfolding around them - with little understanding and consequent lack of control. We're left to interpret the body language (of which 70% of communication arises) and the odd words that are similar in both languages. Just as we would in the same situation. shudder

I've only seen the show as the DVD box set, so the original showing may have been different, but some of the scenes are subtitled, and some aren't. This doesn't seem accidental, or sloppy - like everything in the series, it has meaning and it adds to the development of the story.

The most powerful examples of this are two scenes with the Cousins.

At the end of S3 06, Gus meets the Cousins in the desert, in a very tense confrontation. It's not subtitled, but it doesn't need to be. We know the Cousins want to kill Walt, and we know Gus needs him alive, at least for the time being. They argue about it, Gus says something that makes them think... then he says Hank's name, very clearly. And we know that he's given them Hank, to kill, instead of Walt, and the credits roll.

The very next scene (the first scene of S3 07) is subtitled throughout. The Cousins, as small boys, squabble over a toy, and one (Marco) runs to their uncle (Hector) to complain, and says he wants his brother (Leonel) dead. Hector calls Leonel to him, and plunges his head into a barrel of water, holding him there, asking Marco if that's what he wants. Marco pummels Hector desperately until he releases Leonel, and asks him tearfully if he's all right. Hector tells them: 'Family is all'. Not only does this tell us why the Cousins, and the cartel, are such implacable enemies... it also horrifies us to see the innocence of two little boys who we know will become utterly evil and remorseless. But the entire scene would be incomprehensible without the dialogue, so the subtitles are essential.

You have not really said who "they" is, but on the Breaking Bad Wiki as well as the Blu-ray they are shown.

I have the DVDs and watch them with the subs on, and some of it is important. The setup for Danny Trejo's character's murder was somewhat important, or at least allowed you to understand it better.

The "experiencing the world through Walt and Jesse's eyes" bit is BS. If Jesse and Walt aren't even in the scene, what would it matter? And, like I said, some of the conversations are important to the plot lines. It's not like they're reading the weather report or something, the dialogue does matter.

As for why the original episodes weren't subbed at the time...? Well, there are plenty of Spanish speakers in the U.S. now. I guess they figured half their audience would get it and half wouldn't. It always kinda bugged me that there's a SAP setting for your TV, but no way to translate Spanish to English.

Note that the Season 4 (so it's not the cause of your specific issue) Blu-ray has a glitch where several Spanish-language scenes are supposed to contain English subtitles, but the subtitles don't appear unless you manually enable them through the menu.

Rather than being burned into the image, these subtitles are just another subtitle track, and it's supposed to be enabled by default, so that even when you turn the subtitles "off" altogether, these subtitles appear for the Spanish-language scenes.

Due to the bug, the subtitles do NOT appear if you simply put the disc in your player and hit "play." Instead, you have to enable the full English subtitles via the menu, then turn them off again. The English subtitles will then appear for the Spanish-language scenes as they should.