They have sent letters to people using "photoshop" as a verb, especially in a general context where it's not clear that Adobe Photoshop is the editing software.
Bic (pens) write to stop people using Bic instead of ballpoint. (Very successfully it seems, that's not a use I'm aware of.)
Hoover try to stop people using hoover as a verb too.
Google used to try to stop people using "google it" to mean "web search it".
They have to try to protect the mark, otherwise there's no point them having it.
But it can lead to the loss of the trademark if it becomes a common word. Anyone can brand their product with it.
This happened (or at least went to court) with Hoover in the UK I think. Hoover had become the generic term for a Vacuum Cleaner and so any company could make and sell products described as Hoovers (or at least it was an open legal question).
This means it is far from crazy to defend a trademark from generic use.
There is no danger of these companies losing their trademark, anymore than Apple the iPhone maker is in danger of losing its trademark either because the term Apple is associated with other commercial products, e.g., Beatles records or because it is also a common word.
The companies are worried about loss of "uniqueness" (aka. "trademark dilution"), which is a special form of protection that famous and unambiguous trademarks get. Now that the Google trademark is considered unique, you can't start a Gourmet Google Goulash restaurant, even though there is no risk of consumers being confused about who they are dealing with. The law is worried that Google's brand strength might be diluted by their brand managers not having the stage completely to themselves.
I think this whole area of law is fairly recent - Wikipedia says it grew out of anti-cybersquatting efforts in the mid 1990s.
Trademark dilution is different. You can lose the ability to enforce trademarks that become the generic word to refer to things, on the reasoning that people shouldn't be able to have monopolies over the basic usages of language.
This is why average consumers can't tell the difference between an "iPhone" and a Samsung phone -- they use the name of a popular instance of a thing to refer to all instances. A soda is a Coke, a tissue is a Kleenex, a photocopy is a Xerox, an acetaminophen is a Tylenol, an ibuprofen is an Advil, etc.
For obvious reason it went no-where.