The lowcost pier is shared between Schengen and non-Schengen destinations; when a Schengen destination is in question, the "M" gate is used; when a non-Schengen destination is in question, the corresponding "H" gate is used. There is slightly different infrastructure in use (similar to the D pier, part of which is also shared between Schengen and non-Schengen, and for which these gates can also be "swapped over" to one or the other).

As the rest of that part of the airport is non-Schengen, when you arrive at a "H" gate, you can walk through to the rest of the non-Schengen side of the airport.

However, if you arrive in the lowcost terminal at an "M" gate, there is no airside connection to anything other than other "M" gates...because the rest of the Schengen zone is remote from this pier. (There also is no need to provide an airside connection for M arrivals, as all users of the Low Cost Terminal don't sell connecting itineraries, anyway). By definition, anyone arriving at an "M" gate has Amsterdam as their final destination...so it is not unusual for these passengers to be required to enter the country, even if they are "self-connecting" (If your onward flight is from M, you wouldn't have to).