Because the charger has internal protections both in terms of heat and current capacity that protect it. A light switch is only protected at the breaker box, not at the switch itself like a Battery Minder. To create massive amounts of hydrogen gas with an AGM battery you must charge it at very high rates, well above what a Battery Minder delivers. A Battery Minder fuse is set to blow at 15 amps, its normal output is 4 amps which will not cause hydrogen gassing at that charge rate. To gas out an AGM you have to charge at rates above 30 amps which is waaay above what the transformer on a Battery Minder can even generate.
AGM batteries are "recombinant". From the Concorde web site.:
Flooded lead-acid batteries have excess acid in each cell that prevents recombination of gas during charge. The gases generated during charge (hydrogen and oxygen) must be vented from the cell to avoid pressure build-up. This gas generation depletes the electrolyte of water so periodic replenishment of the water is required. AGM batteries have a “starved” electrolyte condition in each cell that allows recombination of gas during charge. Therefore, replenishing the electrolyte with water is not required.
"During storage, charging, or discharging, RG Series batteries will release a small amount of hydrogen gas. The hydrogen gas must be diluted to prevent the possibility of ignition and/or explosion. Because the rate of hydrogen emission is very low, only minimal airflow is needed to dilute the hydrogen to safe levels. Most aircraft battery compartments have sufficient natural airflow that a separate air ventilation system is unnecessary. When the natural airflow is not sufficient, RG Series batteries are available with vent tubes for connection to air ventilation systems. Contact Concorde if further assistance is needed to determine the airflow requirements."
All this adds up to if you keep the charger below the battery and outside the battery storage area, there is no explosion risk because hydrogen rises and there is no significant hydrogen build up with an AGM battery due to its recombinant design. What little hydrogen there is, is quickly dissipated.