Retiles learn to accosiate the cage opening with 2 things usually.
2. Being taken out
In that order is their concern......feeding inside the cage also leaves the smell of the rat/mouse/rabbbit.
Pythons,boas, and pit vipers use heat sensors on their lips along with smell and vision to locate warm blooded prey. If a snake learns that food comes into its home on a weekly basis, they learn that the vibration from the cage opening is usually followed by food.
One day, it may feel the cage open....smell remaining urine/blood in the cage along with the mouse/rat/rabbit fur.....then see ur hand coming in (roughly the size of a rat), and sense warm blooded creature.
Those 3 things can say "food" to the snake who may bite.....this may hurt and traumatize the keeper, but MORE importantly confuse/traumatize the snake.
Now, if we are talking "hot" or venomous snakes, then I understand feeding in their cages...I've had my share.
Other issues are....substrate. if kept on pine shavings, or bark.....this can be ingested by the snake easily.
More than 1 snake per enclosure at feeding time is a huge no-no
Understand about the venomous snakes... Had a couple of snakes in offices that fed their pythons in the cages. Where do you put them instead? floor? another cage?