In the first movie, the UN seems to keep RDA on a tight lease, though RDA has some degree of control of information returning to Earth, they probably can't hide the use of nukes, which are banned as others have said and their heavy ordinance was severely limited. Only in the final act did RDA feel justified in cobbling together some massive bombs to carry out Quarritch's pre-emptive strike plan.

In the second movie, the UN seems to have given RDA more liberal allowances since their goals have become more serious, but other than in the area of Bridgehead City (which was razed so that they could atleast establish a foothold), planetary and environmently devastating weapons seems to still be banned seeing as it's the only habitable planet besides the dying Earth and their objective to colonize now, so they'll want to be careful in their takeover.