Currently they can't. From IOTA #tanglemath channel I've received an answer similar to this:
Tiny IOT devices like temperature sensors will connect to a proxy node and send the data. The proxy node will take the data and create a TX by doing the POW.
At a later time the sensors will be able to generate the POW themselves by using the JINN processor. This will be a trinary processor with extremely low power usage. Apparently they work since 2 years on that hardware, with CFB/BCNext as the chief architect.
Also remember IOTA is not a decentralized cryptocurrency.
Currently, and since 2 years back, the PoW done on the permissioned network is for illusion of security and marketing. It is the central authority called Coordinator node, which determines the state and consensus.
To be on topic: No coin can do what you ask of today, the most promising is Byteball.
This is not true. The coordinator is an optional compotent. You can configure your node to be part of the network without the coordinator. At the moment nobody is doing it, but it would be possible at any time. If done the network would work without the coordinator, but be suspensible to a 34% attack, just like BTC and ETH are today (51% is an urban legend).
I don't think Byteball is promising. Byteball is basically a clone of IOTA (the author says IOTA inspired him) with added fees, minus the coordinator, minus oracles, minus trinary math, minus the JINN processor, minus a scientific nonprofit foundation.