Why do you feel it is a bug? A random event does random things. If you don't like them, make them rare.

Why is increasing your sensor range stupid? The AI will get the same event. Those are galaxy wide events. Plus, if you built Eyes of the Universe (the last sensor tech), all your ships will have 15 sensor range permanently.

Why bother to research sensors, why bother to build some when any ship have sensor range of 15 from the very beginning, making the complete line obsolete. It does not make sense,

That would be the point of the random event: You now don't have to waste weeks researching all the sensor techs. The AI never researches sensor techs anyways, so if a human player does, it puts them weeks behind. Now you're all on equal footing. The beginning of the game is the best time for that even to happen. You don't have to build scouts to find resources or anything, so your military production can be spent on more important ships likd freighters, colony ships, and constructors.

Also, a random event that only increases sensors by 1 would be rediculous. The random events are supposed to be uber bonuses, hence the 15. Look at how much population growth or influence are increased when those random events happen... A huge galaxy can go from being half grey because of empty sectors to completely filled with racial influence.

This seems a rediculus event to me as it happens when I already have Eyes of the Universe making that acheivement totally not worth the effort put into it anymore.

and if the ai never build sensors and can see anything all the time, thats crap. but it would explain why ai civ survey ships always frly directly to an anomaly in the center of my territor, but never been there before. weird. got to be changed

Use the auto servey option on your survey ship. It detects anomalys.

I hate this one too. It always seems to happen within 5 turns of me building Eyes of the Universe, which makes it worthless and you can't replace it with something else.

I too have never seen this event...

...but the OP will have kittens if he ever tries to play as Neutral.

Edit: Incidentally, I should point out that the main purpose of Eyes of the Universe is to show you ships that are hidden by fog of war. That may not be a big deal on smaller maps, but it'll save your butt in a Gigantic galactic war.

Playing Neutral makes a complete line obsolete, too, by that logic, and it's a much bigger line. It takes weeks to research Terraforming and a few more weeks to get everything terraformed, but you can research the sensors line in one week. I'll admit that Neutral alignment is actually too powerful (though Evil would be better if Mind Control Centers actually worked), but I don't see how an event that advances your research one week is "too powerful" when it's as likely as an event that gives you several dozen new PQ 4-16 planets (yes, that actually happens).

Now, the above assumes that your arguments make some sort of sense. In my experience:
1)Sensors isn't big enough to be a line. That'd be like calling Neutral Shipping a line. Sensors is a sub-line, so that event doesn't actually make a "line" obsolete, only part of one. Also...
2)The Eyes of the Universe is a lifesaver during a war in larger maps, I want it for the ship tracker anyway so that event you're talking about doesn't make Sensors IV any less useful. AT ALL. (See 3.)
3)Sensor range makes exploring too easy? You must be playing on a dang small map. The only things that ever do me any good in the maps I play--the big ones--are engines. I don't even NOTICE expanded sensor range in most cases.
4)You can trade the sensor techs to an AI in exchange for something you need without it helping them against you later in the game. Sensors are great for that.