I think, as someone else mentioned, that this is a 'conservative' approach from the manufacturer in order to make you aware of certain situations that can arise when certain conditions exists - such as overloading a 30a shore power BREAKER, or your internal breaker(s).
In most situations, this is not a concern since the coach is built to handle normal usage of electrical requirements, such as air conditioning, water heating, battery charging, etc. by the generator or with full 50a shore power connection, even with multiple uses at the same time.
When you adjust 'down' to 30a shore power, though, you may need to be aware of what electrical supply requirements are happening in the background that could impact 'over usage' of the available power. With 50a service, you actually have a Maximum of 100a of power, but with 30a you ONLY have 30a - that's less than a third of 'normal'.
If you arrive at a campsite and plug into 30a shore power, your battery Charger may be requiring a lot of amps to recharge your batteries, your water heater electric option may be in demand, and someone may want to use the microwave while the air conditioner is on. This would probably amount to MORE than 30a, and would trip a Breaker. If the 30a breaker is not located at the power pole, you may find yourself hunting the whole campground to find it.
The owners manual is really telling you to be aware that the Charger will probably be requiring a large amperage draw when you first plug into shore power and you may not have enough available power to run large appliances, like air conditioners, especially in conjuction with other items.
One option is to just turn OFF your Charger in this situation, and only turn it on when you are several hours from leaving the site, or overnight when few other items are demanding power...
If you have a 'Shore Max' setting on your Inverter/Charger, you can set it lower to 5a or 10a so that the charger only uses that much maximum power during the charging process.
: ) Fun, ain't it!?!!?