The question is the term "enter." The intent is when a creature physically moves into the area, including involuntary movement (such as the spells you mentioned) it will be affected by the spell. If you instead assume that "entering the area" includes when the area enters a creatures space, this increases the effectiveness of the spell considerably. I'll use Moonbeam as an example.I just checked, both spirit guardians & moonbeam say for the first time on a turn. wall of fire wall of ice cloudkill incendiary cloud cloud of daggers & so on all have a similar if not identical phrase. On top of that, nearly every one of those spells is concentration. You seem to be making a case to safeguard against a situation that's not an issue. some of these spells have ways of moving them (ie moonbeam' bonus action, spirit guardians centered on caster, fog spells & wind, etc) while there are a wide array of ways to force an opponent into their effect ranging from the mundane shove to various magic spells where moving an opponent is the point of the spell.
You cast Moonbeam on an area with 4 creatures in it. By RAI nothing happens to them directly until the start of their turn, but by assuming they "entered" its area this turn, you do damage immediately. Then at the start of their turn, the creatures take damage (and possibly shapechange back) and presumably move away. By RAI, on your turn you can move the Moonbeam, presumably over the same or different enemies, and nothing will happen until the start of their turns (where the cycle repeats itself). By assuming the it moving over a creature is "entering" it for the turn, when you move the Moonbeam, every creature you move it over takes damage on your turn in addition to those who start their turn within in. This effectively doubles the amount of damage this spell does, which is why the RAI clarification exists.