I think most prescriptive sources with an opinion on the matter would consider this to be a hypercorrection. The logic is that the clause must have a subject, and in the absence of anything else, "whoever" is deemed the subject. The hypercorrection probably occurs because:
- "whomever" is an incredibly rare word, and the choice between "who" and "whom" is probably no longer part of the naturally acquired syntax of English;
- there are other, clearly grammatical, instances of "exceptional case marking" where the subject of a clause takes on the objective case due to influence from outside the clause ("they hoped for him to be available").
If you're ever wondering whether to distinguish between "whoever" and "whomever", I would suggest a really simple rule:
Don't ever use "whomever": it's a pointless and old-fashioned.
I would suggest not getting bogged down in spurious debates about "subject" and "object" here: it's nowadays perfectly natural English to say: "I'll give it to whoever", "Pick whoever", "Pick whoever you like" etc.