Article 101 of TfEU states unequivocally that "any agreement to limit or control production, markets, technical developments or investment" are prohibited. When you say that "if you don't want to enter UEFA sanctioned competition you can spend what you (like)" you are actually admitting that if you DO want to enter UEFA sanctioned competition you have to agree NOT to spend what you like, and this is a clear acceptance of a limit on investment. UEFA is quite clear on what it will not let you spend. This is, of course, the fundamental part of FFP, the break even rule; clubs may not spend more than what they " earn" from certain sources. In other words a club may only spend what other companies put into your club, not your owner! If they do the owner/shareholders may only make up 30 million euros of the difference over three years. Now expenditure on academies, stadia etc are not counted towards the break even calculation, though interest on loans to pay for them is. Money spent on players (transfer fees and wages) is counted in full. Money spent on players is investment in an asset just as spending on an academy or a stadium, and it is clear that a team of low cost, poor players will keep an expensive stadium near empty. Thus FFP clearly limits investment by owners and shareholders and, in short, that is what is illegal about that., and further proof of this comes from the settlement agreed by City and UEFA in 2014. UEFA decreed that City's wage bill must not increase for two years, they set down a limit on what City could spend in the transfer market and City were even allowed a squad of only 21 in the CL instead of the 25 other teams were allowed. In no other area of economic activity is this degree of interference from an external body contemplated, let alone allowed. UEFA is in fact abusing a dominant market position - and this is prohibited - by trying to force participants to accept an abrogation of their rights as a pre-entry condition. City's acceptance of the settlement of 2014 was only an agreement in that the club's consent was obtained only because refusal would have led to long legal proceedings damaging to its interests.
You may end being shown to be right but sport has loads of these type rules. F1 has spending limits, rugby union has a salary cap per team, even the PL has it's own financial rules which City signed up to, China has limits on it will allow be spent on foreign players, the list of restrictions compared to "normal life" would go on for miles.
If you get what you're looking for the implications are enormous for every sport on the planet.