It doesn't require that they be solidly bolted together - although that looks to be the case on the particular trailer that we are talking about in this thread.
But the deck isn't just floating on any small trailer. The deck and the axles have to be connected together in some manner so that you can pull both by pulling the other, and so they can go down the road together.
Being connected, the entire axle assembly becomes a beam loaded in the middle by the weight of the log and the vertical component of the chain tension over the log.