If a snake swallows substrate while eating it can cause some pretty nasty issues but if your snake is on newspaper or substrate isn't a factor, feeding in the tub is fine.
Every person has a preference with how they feed their snakes, though. I feed mine out of their tubs in separate bins because they're on a substrate, but I also don't want the scent of delicious rats left over in their tubs and it's easier to keep an eye on them just in case. Plus they're used to it, and I have a system going that just makes it easier to clean and whatnot all at once.
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