[quote=@The Mighty Hero]
[@JBRam2002] They are prone. I was under the impression that to get up from prone would either cost them their free action or half of their movement speed for that turn (I've had DMs who had it do both simultaneously and that seemed highly unfair). The other reasoning  (on my part) for it requiring a free action is that they are in a confined space so limiting their movement doesn't seem like much of a disadvantage. 
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In 5e, there isn't really a limit on the number of free actions you take, as long as it makes sense. You can only take one free action of a single type, however. So using a free action isn't a penalty at all. The movement penalty is the only penalty listed in the PHB for standing from prone.