Is it necessary to strengthen the airframe of an aircraft that is used for sustained low level flight?
Since the question you're ostensibly asking is "Do you need to strengthen the airframes of aircraft used for sustained low-level flight" let's leave aside the military aircraft in your question and just look at "sustained low-level flight" in general.
But first let's bear in mind two concepts:

The first concept is that all aircraft (indeed, all structures) have a finite fatigue life: through normal use, vibration, operating stresses from takeoffs/landings/maneuvers, etc. an aircraft accumulates a certain amount of fatigue, and at some point that fatigue will lead to failures (bending or cracking) in the aircraft's structures.
Aircraft are engineered to withstand a certain amount of fatigue (the "fatigue life" of the airframe) based on a number of factors, and when the fatigue life limit is reached the airframe is either retired (scrapped) or rebuilt/reinforced to extend the fatigue life.

The second concept is something in fluid dynamics called a boundary layer - the region of a fluid in contact with (and most influenced by) some surface.
For purposes of flight we can consider air (the atmosphere) to be a fluid in contact with the Earth's surface. This means down close to the Earth's surface there's a boundary layer with associated meteorological phenomena.
The first article in this NOAA piece talks about some of them, but for purposes of this discussion let's just reduce it to "It's really bumpy and gusty in the boundary layer. Once you climb above the boundary layer the air is smoother."



Bearing these two concepts in mind, an aircraft that spends most of its time operating down load (near the surface of the Earth) is also spending most of its time inside the boundary layer, where it gets bounced around and shaken up more by the turbulence in the boundary layer, and thus accumulates fatigue at an accelerated rate.
Because of this aircraft that spend a lot of time in the boundary layer are usually "overbuilt" to handle a greater degree of fatigue/stress, or have reduced service/fatigue life limits to ensure the aircraft is inspected and repaired/retired before fatigue can lead to a failure.

The accelerated fatigue from spending a lot of time in the boundary layer can also be dealt with by inspections rather than engineering changes. Piper Service Bulletin 886 highlights a particularly dramatic case:


  On March 30, 1987, a PA-28 engaged in pipeline patrol operations suffered an in-flight wing separation resulting in a fatal accident. Investigation revealed the wing failure was due to propagation of a fatigue crack, which originated in the wing lower main spar cap.


Piper then goes on to describe "Normal" and "Severe" usage of the PA-28 family of aircraft, broadly speaking Normal is flying "normally" (Take off, climb to an altitude generally above the boundary layer, fly to your destination, and land), while Severe usage involves spending a lot of time in the boundary layer ("below 1000 feet AGL"). They then recommend inspection intervals based on the type of usage the aircraft is seeing (around 60,000 hours for "normal" usage, and roughly half that for "severe" usage due to the accelerated fatigue accumulation).



TL ; DR

Most aircraft that spend most of their time at low altitudes (close to the ground) are "overbuilt" structurally because there is more turbulence down low, so they accumulate fatigue at an accelerated rate.
Plenty of "normal" aircraft also spend a lot of their life at low altitudes, and this is usually handled by inspecting them for signs of fatigue damage more frequently than their higher-altitude counterparts.