They want to make the stickers difficult to remove to discourage moving them from vehicle to vehicle. It is a global frustration.
There are many blog posts about removing parking decals, inspection cards, and other stickers from the inside of a car windshield, including the question How do I remove old stickers from my windshield? at Mechanics.SE. The accepted (and solitary) answer there states
For vinyl stickers I've had good experience warming them up with a hairdryer to soften the glue and then slowly peeling them off. I've also managed to get them off a windshield using a scraper … To remove the glue residue I've had good experience with both WD40 (which you can also use to soften paper stickers) and tar spot remover
For myself, living in a place where annual registration and vehicle inspection must be displayed in the windshield and changed annually, this seems excessive; at the very least, I'd need to purchase quite a few extension cords to run a hairdryer out to my car.
Method 1: Scraper
The razor blade is a commodity item on this side of the pond, and can be purchased in any home improvement store and even many supermarkets and pharmacies. The metal is softer than the windshield glass, and so it is difficult to scratch the glass if you are careful.
Especially for vinyl stickers, the adhesive may be loosened with heat. Running the car's defroster for several minutes helps.
Use the razor blade to loosen a corner of the sticker and work it through the adhesive with very short strokes. Once you have pried up enough of it to grip with one hand, peel it back with slight pressure and continue using the razor blade to pry up the remainder of it.
When all of the sticker is removed, I use the same blade to scrape off the remaining adhesive. If the remaining film is very thin, sometimes it is more easily rubbed off with a washcloth and ammonia-based cleaner or isopropyl alcohol.
This is the only method I use as I do not trust myself not to drip a solvent onto the dashboard or elsewhere in the car.
Method 2: Solvent and scraper
The above can be a tedious process, and you can speed it up mightily applying a solvent with a washcloth to dissolve the adhesive. Which kind is most effective depends on the kind of adhesive, naturally, but commonly suggested solvents include
- Acetone (found in nail polish remover)
- Degreasing products containing citrus oils (e.g. "Goo Gone")
- Hot water
- Strong vinegar
- WD-40 or another penetrating oil
- Baby oil or vegetable oil
- Peanut butter
These can be messy, and some of them are highly corrosive/irritating. A single drip onto the dashboard may discolor or damage it permanently, and fumes may linger long after the removal process is completed. Moreover, you may accidentally get some on stickers that need to stay put. Use at your own risk, in a well-ventilated area, using gloves or other appropriate safety gear.
With the weaker solvents, it may take some time to dissolve enough adhesive to be useful; for peanut butter, some say to soak overnight. Once you are ready, a small amount of dishwashing liquid or another light detergent may help break up the oil-imbued adhesive. Ideally, you might be able to peel off the sticker without scraping, though you may still find a razor blade helpful to peel up a corner to get started.
3. Method 3: Don't get stickered
One "hack" for self-applied stickers is to use a phone screen protector; apply the sticker to it, then apply the protector to the windshield. You can trim off the excess, and since the protector is transparent, you should be all right with the authorities— or at least escape official notice. Being designed to be semi-permanent, the adhesive on the protector is strong but not impossible to remove.
I cannot say as to whether some jurisdictions might require windshield stickers to be "permanently affixed" or some such, and whether this would pass muster. And of course, there are cases where the sticker must be applied by the inspector/official/mechanic directly, in which case you have no choice. But in cases where you're acquiring and applying the sticker yourself, this would seem convenient and inexpensive. There are some commercial products ("sticker shields") that may be slightly cheaper, but these days a smartphone screen protector is very easy to find.