I don't see how providing unmetered Spotify is "forcing" any Apple Music / Google Play Music subscriber to switch. You can freely choose to ignore the unmetered services and continue to use whatever other services you choose.
You have three competitors: A, B, and C. They all pretty much accomplish the same thing. Your ISP cooperates with A to give it a privileged position (unmetered connection).
If you're a subscriber to B and C, you can't use your service of choice under the same conditions as those who use the service A. Therefore, you can either switch to A, or pay more data to use your preferred service (B or C).
As long as you can't choose the service that's going to be unmetered, Spotify has a privileged position compared to its competitors for the users of that ISP.
For your mentioned services B and C for which you are subscriber, you have been paying for the data usage, you are free to use them in the future with the exact same conditions/charges. If A enters some agreements with the ISP/carrier you choose to give you unmetered access, you are not put into any disadvantaged position, because you are not paying extra, you only get an option to use A's free traffic or pay the current same amount for services from B and C.
It should also be pointed out that services A, B and C you described above are not "pretty much accomplish the same thing", A managed to reach this agreement to foot a part of your traffic bill, B and C refused to somehow pay your carrier to match the same level of service currently offered by A. There is a huge difference here.
Any ISP would be free to sign an "exclusive" agreement with a Spotify and B+C are now stuck having their customers charged more without any ability to control the price or do anything about it.. Or the exclusive agreement could be with a carrier-owned service and A, B and C are all stuck.
Unless the ISP is downgrading the service quality of B/C or charging you more for using the same services from B/C, I don't see any problem if they choose to make services from A unmetered. The entire argument is so flowed - using such logic, one can jump up and argue that unmetered Spotify traffic is bad because people are "forced" to listen more music and spend less time on sports/reading/<insert your favourite stuff here> related apps/services - what is the difference here, they all compete for your time/attention.
It would be a net neutrality issue if the ISP blocked B and C as services so you were forced to only use A if you wanted music streaming. Or similarly, if music streaming services were penalised for bandwidth to the point of being unusable.
For me, net neutrality isn't about not making one service better. I have no particular issue with that. It's about not making services unusable.
How can you be sure? It could that the costs of the unlimited data for A get apportioned on all paying customers.
Because that's exactly what is happening.
Except tomorrow they will limit A as well.
Internet is never free.
When people finally realized how much they were getting ripped off - they figured communicating via the internet via platforms like WhatsApp and Messenger is much cheaper.
The carriers took a massive hit to their bottom line - and are now trying to get it back via implementing the same practices onto internet services.