There IS NO law that says herbicides have to dissipate after a few days. In general the US compared to other countries has very lax laws about poisons and hardly any enforcement.
Weed n' feed is probably NOT RoundUp/glyphosate. It is a combination of a synthetic fertilizer, 2,4-D, and mecoprop-p.
Here's a thread where I collected a bunch of information about commonly used garden poisons: viewtopic.php?f=11&t=57653
Here's part of what I found about 2,4-D:
Consumption of plants treated with 2,4-D has killed horses and cattle. It can be toxic to fish and aquatic organisms if it leaches in to rivers. 2,4-D is slightly toxic to wildfowl (mallards, pheasants, quail and pigeons) and moderately toxic to other birds. In birds it has been shown to cause birth defects and reduce hatching success. It also indirectly affects birds by destroying habitat and food sources. Moderate doses of 2,4-D severely impaired honeybees brood production, reducing offspring numbers. It kills predatory (beneficial) beetles and ladybug larvae, resulting in an increase in aphid infestations. 2,4 dichlorophenol, a breakdown product of 2,4-D is extremely toxic to earthworms, 15 times more toxic than 2,4-D itself.
Fate in the Environment: 2,4-D is not very persistent in the soil, with a half-life in the soil of 16 days or less. 2,4-D has been included on the EPA list of compounds that are likely to leach from soil. Despite its short half-life in soil and in aquatic environments, the compound has been detected in groundwater supplies in at least five States and in Canada. It has also been detected in surface waters throughout the United States at very low concentrations.
Note that half life of 16 days means that in that time period the amount of 2,4-D has been reduced by half. In another 16 days it will have reduced by half again, so 1/4 of the product will be left. AND it doesn't mean it is gone. It means it is no longer identifiable as 2,4-D, because it has broken down into component parts. But as noted, sometimes the breakdown products can be worse than the original poison. 2,4-D has been banned in Australia, Canada, Europe and other places...