Not being advertised for or primarily used for copyright infringement.
Google Music asked me to upload songs and I did. Is Google really going to get sued to oblivion? I doubt it.
In the UK its technically illegal to rip a CD to your own PC to back it up, for example, see the recent high court case(s) about it where the government won against Brennan (who make hifi gear which can do this) - no different to this app.
The same can't be said for OP who probably doesn't have the time or money to fight it as evidenced by this post.
Part of the reason google/youtube are quasi-monopolistic is that they've already been through all of this, they've had the arguments and the lawsuits in various different territories, and they've reached agreements and paid money to deal with the copyright issues.
> According to Capitol Records, Inc. v. MP3Tunes, LLC, this is absolutely OK in the eyes of the law
https://casetext.com/case/capitol-records-inc-v-mp3tunes-llc
From a quick read of that, it appears that Mp3tunes lost the case, had a huge damages award made against them, as happens a lot, and the overall damages were deemed excessive and reduced to $750k.
In particular is said that the DMCA safe harbor is available to such services, and that deduplication is not a problem.
MP3Tunes did not adequately deal with DMCA takedown requests so didn't get full safe harbor protection leaving them liable for some user's uploads. Also MP3Tunes' founder had infringing songs stored and they got nailed for those.