Have you ever tried carrying/concealing a grinder as an offensive weapon? It's really not that practical, but not half as impractical as carrying the generator you need to power it in the street!!
The sale of aerosol paint is restricted to over 16's is nothing to do with safety, or offensive weapons, it's to prevent them being used for graffiti/vandalism by minors under the Anti Social behaviour act 2003. The sale of aerosols containing solvents is actually restricted to over 18's though, again this has nothing to do with safety or offensive weapons, it's in order to reduce solvent abuse amongst under 18's under the Intoxicating Substances Supply Act 1985.
I understand and know why aerosols are restricted as I worked in retail for 6 years. What gets me is that they are restricted (and rightly so for the reasons you stated) yet power tools aren't.
You can buy battery operated grinders and drills just to point out. You can buy those battery operated spray machines and a can of paint if you really wanted to vandalise something yet they aren't restricted.
I don't see the being able to conceal the item relevant because hedge shears are restricted and they aren't exactly easy to conceal. I personally think tin snips are just a bit of an oversight really. They probably will be restricted at some point.