Exactly. You can’t spam upvites, because the problem with spam is that it buries and dilutes actual, useful, quality content. A user “spamming” upvoting or downvoting isn’t really actionable spam - his up/downvote isn’t drowning out the others.

The problem is brigading - when multiple people start mass up/downvoting and water down other votes.

Same goes for spaming votes with multiole accounts.