Why Switching amplifiers (Class D, E and F) drive the active devices fully on or fully off, so the raw output is essentially a square wave rather than a sine wave. A square wave is loaded with harmonics of the operating frequency, so a low-pass or band-pass filter (often a resonant tank) must follow the amplifier to strip those harmonics and leave a clean carrier. Without it the amplifier would violate the spurious emission limits of 97.307 and splatter energy across other bands.
Watch out A high-pass filter is exactly backwards: the problem is energy above the operating frequency, not weak low-frequency response. Load resistors do not belong there either, since a switching amplifier gets its efficiency by delivering all its power to the antenna, not burning it in a resistor.
Switching amp = square wave out; square waves are full of harmonics, so always follow with a low-pass filter.