Should corporations like Google be legally allowed to force their users to use their other services?

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.