Is this even against their policies? It strikes me as a hard policy to write. Compare it to "write and distribute a whitepaper about XYZ". That certainly sounds like a legitimate task, but it is only different from this example in scale.

At least if they decide to to forbid this practice, they can pay their own turksters to find and flag the forbidden activities.

Mturk > FAQ > Policies: You may not use Mturk for spamming activities. (https://requester.mturk.com/mturk/help?helpPage=policies)

Workers may flag any HIT they feel violates Mturk policy. Flagged requesters risk getting banned by staff.