Although the laxative action of magnesium is thought to be due to a local effect in the intestinal tract, it releases hormones such as cholecystokinin (which stimulates the biliary system) or activates constitutive nitric oxide synthase which also contributes. Under normal circumstances high doses of oral magnesium salts is safe and some salts, such as magnesium hydroxide and magnesium oxide also have an antacid effect. The concern is that prolonged and repeated use of any laxative of this type including Epsom Salts may allow sufficient absorption into the systemic circulation to cause renal or other organ toxicity: albeit temporarily. That is one reason why it is recommended that the kidneys are working as well as they can prior to a liver-flush.
I consider that the very small concerns are well worth trading off against the health gains. It amuses me that such a song and dance is made about these very safe medicines but warning labels aren't put on fast food meals! We all have to make choices on what is best for us based upon sound research and we don't need approval for these decisions.