Is it because Chespin seems to be physically oriented? If it is, then remember that Solrock and Sawsbuck learn Solarbeam through level, but both have higher physical Attack than Special Attack and Ammonguss has equal attacking stats.I dunno, I can't imagine chespin learning solarbeam level up.
And I'm happy orotto is finally revealed.
Or is it because you don't think they would let a starter learn two STAB attacks with a base power of 120 by level? If so, Venusaur learns both Petal Dance and Solarbeam by level.