Thing of it is that there are other search engines, Google is just the elephant in the room. Google is a private corporation and as such is free to implement any feature that it cares to insert into its product. If users don't like the service, they are free to go elsewhere. If a large enough number of users stop accessing Google, the company will change its viewpoint or go out of business. That's the way a free market works. Be aware that Google has become so large that it will take quite a long time for the execs to notice that their userbase is down and even longer to react but react they will. Money talks loudly in the ears of greedy company executives.
Google, over the time, has forced their YouTube users to use various other products, like Google+ or Gmail. Should such tactics be legally allowed?
For one thing, it's the very definition of anti-competitive behavior. It's bundling of services in a way that allows them to leverage their market advantage. Even in theory, markets only work if there's intense competition in each separate market. But the moment one actor can leverage their strength in one market into another, competition goes away and the advantages of economies of scale become insurmountable.
Worse, it's just impossible to negotiate in good faith in this way. EULAs should already be unenforceable in a lot of ways: many are for products you purchase before you sign anything, which is the definition of an unenforceable contract since it is requiring someone to negotiate to use something they already own meaning there is effectively no consideration at play, and most are so labyrinthine and include so many terms that can be changed any time in perpetuity that there's simply no possible way that they could be negotiated in good faith. Insisting that someone simultaneously agree to several of these already-questionable EULAs is laughable.
Nobody is forced to do anything. They have replaced the YouTube commenting system with Google+ posts. This means you need to create a Google+ profile, but if you don't want that, then hey, don't comment!
I had a YouTube account before the Google+ change so I get a choice. Like User01834 said you don't have to use the gmail and google+ it is just the door and it's free.