Actually, you will not create 100% pure (i.e. no fallout) thermonuclear (fusion) bomb. Fusion process itself generates stray neutrons (among other radiation types) that can, and will cause secondary radioactive isotopes form from matter around the bomb, thru neutron activation process. While it's not nearly as large amount of fallout compared to same yield of fission, it's still significant. Gamma rays and xrays are also inherently produced during fusion reactions and are both ionising (though these are considered prompt radiation source, not fallout).
You can limit the radiation output of a classic teller-ullam design (together with its yield) by changing second stage pusher/tamper layer (which is part of the weapon that is both used as a - sort of - "hammer" / casing to the fusion fuel, that compresses it into the "anvil" - the U235 "spark plug" in the center of secondary stage) from usually used Uranium-238 to Lead (as i.e. Russians did when they limited the Tsar Bomba from its design yield of 100 megatons to ~50 megatons). As lead does not fission (and indeed, it will actually prevent neutrons from irradiating the area, by a little bit), as uranium does, the yield, and release neutrons/ionising photons/fission byproducts is smaller.
Another thing to consider is, fusion weapons are also considered "clean" because any promptly dangerous radiation is present so close to the hypocenter, that survival is very unlikely anyway, especially for very high yield weapons, range for very high radiation emission might be well inside the fireball.
As for costs, it will be expensive anyway. You cannot just use regular hydrogen in a fusion weapon. Sure, a star can use H-H fusion process, but it is dependent on extreme pressure, that a bomb cannot generate. Instead, thermonuclear weapon provides enough pressures/energy needed for another kind of fuel, namely a heavier isotopes of hydrogen - deuterium (mostly in form of lithium deuteride solid fuel) or tritium (rarely in form of either tritium gas, in fusion boosted fission weapons, or cryogenic tritium). Both are very expensive to manufacture large-scale, and in case of tritium, decay is so fast that weapons using it need their fusion fuel changed every so often in order to remain viable (this, and the need for cryostorage, is one of reasons that modern systems use Li6D fuel. About the only western system using tritium were non-weapon test systems.).
Now, as to why it would not change anything - WMDs are not feared because radiation (which is very overrated and mostly a psychological scare, but if one knows of it, precautions can be taken to limit its effectiveness (aside from prompt neutron/gamma/xray pulse)). They are feared for combined issue of easy delivery (nowadays, with a ballistic missile, you can obliterate any place on earth within 20-30 minutes of launch, with little to none chance of someone stopping the warhead) and sheer destructive power of blast and thermal pulse. While Hiroshima's Little Boy was very powerful compared to classical explosives, it pales in comparison to really heavy warheads (though almost all of really heavy stuff was removed from stockpiles due to international ban treaties).
For example - take the Russian R36M (or NATO name SS-18 Satan) ICBM. One of its warhead wariants is using single 20-25 megaton warhead. That yield is able to cause enough prompt thermal pulse to cause lethal 3rd degree burns at the range of 40 (forty) kilometers from the hypocenter, provided there is line of sight to the fireball at the moment of detonation. Literally everyone that sees the explosion moment (or more fitting, is "seen" by the bomb), will be burned. Badly.
Go closer than those 40 km, and thermal pulse effects are worse. Up to and including your body flashing into steam (and, effectively, leaving a less bleached patch of ground below, as a shadow - as those preserved in Hiroshima) a few kilometers from the fireballs surface (and that fireball alone is 3-4 kilometers in diameter).
And the effects arrive, as I said - promptly. Not within seconds. You are hit by the thermal pulse at the moment you see the explosion (as it's not heated air coming to you, its thermal photons arriving together with the ones in visual range of em spectrum). Buildings will be heavily damaged at about half that range, construction strength dependent.
It's the extent, and literal inevitability of damage, that is being feared, not radiation itself.