@Liu
From my understanding, it's not actual "permission" but more like "formality" or "politeness". Some groups do have official licenses, with groups like AsSlators being given permission by their creators, while others used to release manga in an official capacity, but have since folded and decided to upload the English translations so that they don't disappear forever.
Those are groups that you do not snipe from, at all cases. But for the most part, it's just become an "unwritten pact" from the earlier days of scanlation where there used to be translation teams that would fight over the same comic. I may be wrong, but the reasoning goes something along the lines of:
1) So that the overall translation teams in the world don't accidentally waste their time translating what's already been translated, or what's being translated, so overall, more comics are translated.
2) Reader's consistency. Some groups choose to translate things differently, so reader's may get "translation shock" when the character's names suddenly change. For example, the current translators of Tondemo Skill De Isekai Hourou Meshi call the giant wolf "Feru" while the official translation calls him "Fel", based on the official novel's translation.
3) Avoiding drama.
It's not so much as enforced, so much as it's seen as a dick move. Sure, you could translate the chapter, typeset and everything, but it also means that the current translation team's work is basically down the drain, or redundant. Then, who knows if you'll translate the next chapter too? Is the quality as good or is it just Google Translate slapdashed onto whited out boxes?
Will people be mad? Sometimes. Again, it depends. All the "reaction" to the "sniping" is based on many factors like
"how long was it before the last translation?" (usually around 3 - 6 months of no updates are where people start to see it as free game, with a 1 year gap usually considered by everyone to be open to pick up),
"quality" (the worse the quality, the more people are mad.),
"release consistency" (if it's a weekly comic, or a monthly comic, and the current translators only release a chapter every 3 months, it's technically consistent, but a lot of people would be fine with it being sniped.),
"how did the current translators act during previous snipes" (For example, Not a Scans Scans Group didn't translate Milady Just Wants to Relax for months, and a kind-hearted individual translated a chapter, so Not a Scans Scans Group suddenly releases that chapter, then the next chapter, then sends a message to that individual a somewhat threatening, mafia-esque message along the lines of "Why didn't you ask for permissions, bud?" People were happy that there was a bit of a translation war after that.),
"hoarding" (a lot of translation groups translates one or a few chapters, then stops translating then sits on them because of a lack of desire or resources to keep translating consistently; see the Not a Scans Scans Group above. People generally are fine with the sniping because it sort of cracks the whip.), "
chapter skipping" (with Iruma-kun, while the translators were at around chapter 70 or so, the Russian team translated to like 110, and a translation group decided to translate chapter 110 and that alone, pissing people off.) and more.