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.)
