Why are regulations regarded as mere suggestions?

Your question can be closely followed with.
"Why do many NCOs selectively enforce regulations"

And neither question will have a definitive answer

In the case of the USAF, A Chief of Staff, who will remain nameless, changed our regs into "Instructions". Thus the room to interpret the AFI (Air Force Instruction", which in my opinion led to slack in the spirit of the reg. I suppose all the services have generational slack at some point. Doesn't make it right, or good for task accomplishment and evaluation.

Glad it's not just us!

Because many leaders do not have the intestinal fortitude to enforce the regulations, choosing instead to be "buddies" with their Soldiers.

One example from my unit: headgear. I have corrected many Soldiers outside smoking without headgear. One senior Specialist told me that he "didn't know" he had to wear headgear outdoors. BS. The new guys just followed the example they were shown. I've corrected sergeants (E-5) for the same offense. They use the excuse "I'm just going to my car." No matter. I tell them they are setting a bad example as leaders, doing the wrong thing for all to see. Another variation is wearing the incorrect headgear. At the 82nd, the maroon beret is the standard headgear when not in a field environment or at the motorpool. Yet I see Soldiers walking around wearing their patrol caps because it is more convenient. Wrong is wrong. Leaders who do not enforce the standard or violate it contribute to the breakdown of discipline.

Start enforcing the standard and we might get somewhere.

It's not that they are merged suggestions, the fight is that Leader pick and choice when they what to make corrections and inform personnel of the problem on the spot. This creates new, unrecognizable standards within the Army which soon forms toxic leadership. Leaders think they're being a friend or morally right by not correcting the problem, but they are doing more damage then they realize. As my old CSM use to say, "it's rare hearing someone got bit by a tiger, but you'll hear many tell about the mosquito bites they can't seem to avoid, as it's the little stuff that gets use every time".

In Whose Army... Regulations Are Pretty Much Requirements And Guidelines...

Because not enough leaders actually take the time to read the regulations. You can't enforce what you don't know. I also hear way too many people giving incorrect guidance or spot corrections based solely on what they were told when they were junior, or what their previous unit did with no doctrinal foundation.

Regulations or in the case of the Air Force, Air Force instructions, AFI never have been suggestions, they are requirements. Even a supplement to them can add but not change anything in the original directive. Failure to follow these Regulations or Instructions cannot be justified and is not an option and as long as they are followed and its written there is no question what is required. When people deviate from those standards then no one knows what to expect and even the negative influence on moral and lack of uniformity demonstrates quite well why this is not acceptable and more than change for the sake of change is.

Not in my Marine Corps.