That's a great catch!


In 5E the rules for casting two spells in the same round are exactly as you've stated: the second spell must be a cantrip.

That rule is especially important with Sorcerers.


In 5E Sorcerers have metamagic which allows them to quicken a 1 action spell and cast it as a bonus action at the cost of 2 sorcery points.
If the second spell as a cantrip rule isn't enforced a player could conceivably cast a fireball and then quicken a fireball and cast it as a bonus action in the same round.

That could potentially unbalance sorcerers and give them more nova potential then any other class.



(This of course assuming that metamagic is being implemented.)