(This answer is related to my answer on Bacteria resistance to natural antibiotics on the Biology SE)
First of all: When it comes to evolution, Biology doesn't talk of 'strong' and 'weak' , it talks of 'fitness' ("survival of the fittest", for example), which is about how well adapted to their environment an organism is. Fitness includes how well it can survive in its environment and how much it can produce vital offspring.
This seems logical because in ordinary environment, the wild type of virus should be the strongest one. Any mutation of it is to fight back the drug, hence it is not the strongest one anymore
This is a misunderstanding of evolution. The wild type of a species is not the fittest this organism can be. That it is the wild type just means that it evolved and spread. Something that doesn't evolve can't spread (as an hyperbolic example, it might be advantageous for humans to have wings, but since we didn't evolve it, our wild type doesn't include it).
It also helps to remember that "the wild type" is not a uniform thing. There are many genetic variations which we still consider to be the wild type. Even bacteria that reproduce through asexual reproduction are not all the same, even within one host.
In the natural environment of the bacteria you are talking about, an antibiotic resistance confers little benefit because they don't encounter it. So if it evolves, it won't help these organisms survive and would only spread throughout the population by chance. On the contrary, it might even confer a disadvantage - for example, penicillin resistance required producing the protein beta-lactamase. In a penicillin free environment, this might just be a costly extra without a benefit. Over time, it might get lost.
In a human taking antibiotics, however, these bacteria with an antibiotic resistance are indeed fitter and have a survival advantage. Bacteria with multiple resistances even more so. Between bacteria, these resistances can actually be transferred by something called "transformation" that involves actually picking up genetic material, so evolving multiple resistances also isn't as uncommon as it may seem.
As for your virus example, without knowing more, I'd guess that the fitness reduction in the more resistant virus strain is probably a side effect of the resistance. Getting more or less fit through evolving resistance to treatment is not, in itself, a difference between viruses and bacteria.
Origins and Evolution of Antibiotic Resistance
Mechanisms of Bacterial Resistance to Antibiotics
What is fitness?