Why manure is considered bad or risky for potted plants while they are considered good for plants on ground?
If they are risky or bad, why do the nursery sell them?
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It's a little bit complicated - properly composted manure can be added to potting mixes, but its the 'properly' composted bit where the problem arises. Manure should be composted with other materials for up to two years (horse manure with straw, for instance), and turned regularly so that the pile of composting manure heats up to pretty high temperatures. This ensures that the manure is, as far as possible, pathogen free, and is then safe to use, in ratio with other materials such as loam, sand, peat, etc., in potting mixes. If you're sure that any manure you have access to has been processed properly, then use it, but don't use anything that smells or looks bad.
Manuring open ground is not quite so risky because on open soil, any possible pathogens are diluted, so to speak, and will be diffused widely, which they are not when contained in a small amount of soil in a pot. Even on open ground, though, manure should be composted prior to use.
I don't know what part of the world you're in, nor what products you're referring to, so can't comment on manure based products on sale where you are.
In addition of the answer from Bamboo:
Manure is slow to decompose, so it enriches the soil on long term, but in a pot one probably change the ground more often (one or two years). Also the roots are less developed. So I would use manure for long term sustainability of soil (especially for the P and K components), but not for a pot. Additionally it needs some bacteria to decompose it, which could not be available on a pot. Note: on chemical fertilizers, one could choose the chemical components in order to have a fast or slow decomposition, on manure one has not such choice.
Acidity could cause some problems in the pot (direct or indirect, by reducing assimilation of some minerals). On ground the plants have longer roots, so they will find anyway some less acid ground. (anyway over-fertilizing causes also problems on ground)