A scumbag dealer can try to void warranty (or parts of it) for any number of reasons. From a catch can (or other simple mods) to not getting your oil changes done there. Many have the whole "I am bigger than you" mentality and don't think you'll take them to court over it, sort of a bully mentality.
Good dealers don't care unless they can prove directly your part caused the issue, and even then, good ones usually want to work with you. Trick is finding one, and sometimes the sales department can be great and the service department terrible, or vice versa.
That is why I tend to buy parts from bigger names, rather than smaller names, I hate to say. If the dealer tells me they are voiding my engine warranty because I use Amsoil, I can go to Amsoil and make a fuss and they will go to bat for me, as they won't want the publicity of "Amsoil oil voids warranties." If they tell me they are voiding a warranty due to an AFE intake, I can probably get AFE to go toe-to-toe with them. Etc. Not fool proof, but it helps.
That's exactly right! These are facts! Find the right dealer (it took me twice, but I have it, the best), establish a good working relationship with them, and sales, parts, and warranty associates right from the get-go. Introduce yourself, ask questions, be interested in each of them. Remember folks, "People will forget what you did or what you asked for (or complained about), but they will never forget how you made them feel!" Make them feel important, which they are, and they in return, will take care of you! This is fact!
Spend time in their store when you get your 5,000-mile oil change or whatever. Be cordial, interested, excited, and fun to have as a customer. Keep your six-shooters holstered and observe their shop, it is nice or a get-by raggedy-Ann shop? Are associates happy, rude, or attentive when you walk in and leave? These are all tell-tale signs of a good or bad dealer! Then you can mod all you want within reason, and they won't say a thing. That's how I handle it all...
Final observation: Working for a dealer when customers are in full defense/demand mode day after day, for not getting good product or service, deserves a reexamination, which brings what Chewtoy speaks of. When they're bad, then move on to the next dealer, when they are good, you'll know it.